Flying Fortress B17-G

Origins

On 8 August 1934, the U.S. Army Air Corps (USAAC) tendered a proposal for a multi-engine bomber to replace the Martin B-10. The air corps was looking for a bomber capable of reinforcing the air forces in Hawaii, Panama, and Alaska. Requirements were that it would carry a "useful bombload" at an altitude of 10,000 ft (3,048 m) for ten hours with a top speed of at least 200 mph (320 km/h).

They also desired, but did not require, a range of 2,000 mi (3,200 km) and a speed of 250 mph (400 km/h). The competition for the air corps contract would be decided by a "fly-off" between Boeing's design, the Douglas DB-1, and the Martin Model 146 at Wright Field in Dayton, Ohio.

The prototype B-17, with the Boeing factory designation of Model 299, was designed by a team of engineers led by E. Gifford Emery and Edward Curtis Wells, and was built at Boeing's own expense.It combined features of the experimental Boeing XB-15 bomber with the Boeing 247 transport aircraft. The B-17's armament consisted of up to 4,800 lb (2,200 kg) of bombs on two racks in the bomb bay behind the cockpit, and initially possessed five .30 caliber (7.62 mm) machine guns. It was powered by four Pratt & Whitney R-1690 "Hornet" radial engines each producing 750 hp (600 kW) at 7,000 ft (2,100 m).

The first flight of the Model 299 was on 28 July 1935 with Boeing chief test-pilot Leslie Tower at the controls.Richard Williams, a reporter for the Seattle Times, coined the name "Flying Fortress" when the Model 299 was rolled out bristling with multiple machine gun installations. 

Waist position gun blister of Model 299, not adopted for production

As the production line developed, Boeing engineers continued to improve upon the basic design. To enhance performance at slower speeds, the B-17B was altered to include larger rudders and flaps. The B-17C changed from three bulged, oval shaped machine gun blisters to two flush, oval-shaped machine gun window openings and a single "bathtub" machine gun gondola housing on the lower fuselage, that resembled the similarly-configured and located ventral defensive emplacement on the German Heinkel He 111 P-series medium bomber. Models A through D of the B-17 were designed defensively, while the large-tailed B-17E was the first model primarily focused on offensive warfare.

The B-17E was an extensive revision of the Model 299 design: The fuselage was extended by 10 ft (3.0 m); a much larger rear fuselage, vertical tailfin, rudder, and horizontal stabilizer were added to the design; a gunner's position was added in the new tail; the nose (especially the bombardier's well-framed nose glazing) remained relatively the same as the earlier B through Dversions had, but with the addition of a Sperry electrically-powered manned dorsal gun turret just behind the cockpit, and the similarly-powered (also built by Sperry) manned ventral ball turret just aft of the bomb bay – replacing a relatively hard-to-use, Bendix-designed remotely-operated ventral turret on the earliest examples of the E variant, that had also been used on the earlier marks of the North American B-25 Mitchell – resulted in a 20% increase in aircraft weight.The B-17's turbocharged Wright R-1820 Cyclone 9 engines were upgraded to increasingly more powerful versions of the same powerplants multiple times throughout its production, and similarly, the number of machine gun emplacement locations was increased to enhance their aircraft's combat effectiveness.

The B-17F variants were the primary versions flying for the Eighth Air Force to face the Germans in 1943, and had standardized the manned Sperry ball turret for ventral defense, replacing the earlier, ten-panel well-framed bombardier's nose glazing from the B subtype with an enlarged, nearly frameless plexiglas bombardier's nose enclosure for improved forward vision.

Design and variants

Production numbersVariantProducedFirst flightModel 299128 July 1935 YB-17132 December 1936 YB-17A129 April 1938. B-17B3927 June 1939 B-17C3821 July 1940 B-17D423 February 1941 B-17E5125 September 1941 B-17F (total)3,40530 May 1942 B-17F-BO2,300 B-17F-DL605 B-17F-VE500 B-17G (total) 8,680August 16, 1943B-17G-BO4,035B-17G-DL2,395B-17G-VE2,250Total12,731Notes:
B-17s were built at Boeing Plant 2
Seattle, Washington (BO)
and starting with the B-17F also at
Lockheed Vega, Burbank California (VE) and
Douglas Aircraft, Long Beach California (DL)

The aircraft went through several alterations in each of its design stages and variants. Of the 13 YB-17s ordered for service testing, 12 were used by the 2nd Bomb Group of Langley Field, Virginia, to develop heavy bombing techniques, and the 13th was used for flight testing at the Material Division at Wright Field, Ohio.Experiments on this aircraft led to the use of a quartet of turbo-superchargers which would become standard on the B-17 line. A 14th aircraft, the YB-17A, originally destined for ground testing only and upgraded with the turbochargers, was re-designated B-17A after testing had finished.

 
 

Two experimental versions of the B-17 were flown under different designations, the XB-38 Flying Fortress and the YB-40 Flying Fortress. The XB-38 was an engine testbed for Allison V-1710 liquid-cooled engines, should the Wright engines normally used on the B-17 become unavailable. The only prototype XB-38 to fly crashed on its ninth flight and the type was abandoned, the V-1710 being kept for fighters.

The YB-40 was a heavily armed modification of the standard B-17 used before the North American P-51 Mustang, an effective long-range fighter, became available to act as escort. Additional armament included an additional dorsal turret in the radio room, a remotely operated and fired Bendix-built "chin turret" and twin .50 in (12.7 mm) guns in each of the waist positions. The ammunition load was over 11,000 rounds. All of these modifications made the YB-40 well over 10,000 lb (4,500 kg) heavier than a fully loaded B-17F. The YB-40s with their numerous heavy modifications had trouble keeping up with the lighter bombers once they had dropped their bombs, and so the project was abandoned and finally phased out in July 1943, but not before the final production blocks of the B-17F from Douglas' plants adopted the YB-40's remotely operated and fired Bendix "chin turret" for a much-improved forward defensive weapons installation.

By the time the definitive B-17G appeared, the number of guns had been increased from seven to thirteen, the designs of the gun stations were finalized, and other adjustments were completed. The B-17G was the final version of the Flying Fortress, incorporating all changes made to its predecessor, the B-17F, adopting the remotely operated "chin turret" for forward defense from the YB-40 "gunship" version, and in total 8,680 were built,  the last one (by Lockheed) on 28 July 1945.Many B-17Gs were converted for other missions such as cargo hauling, engine testing and reconnaissance. Initially designated SB-17G, a number of B-17Gs were also converted for search-and-rescue duties, later to be redesignated B-17H.

 

Operational history

 
B-17 Flying Fortresses from the398th Bombardment Group fly a bombing run to Neumünster, Germany, on 13 April 1945. On 8 May, Germany surrendered, and Victory in Europe Daywas declared.

The B-17 began operations in World War II with the Royal Air Force (RAF) in 1941 (but was not successful), and in the Southwest Pacific with the U.S. Army. The 19th Bombardment Group had deployed to Clark Field in the Philippines a few weeks before the Japanese attack on Pearl Harbor as the first of a planned heavy bomber buildup in the Pacific. Half of the group's B-17s were wiped out on 8 December 1941 when they were caught on the ground during refueling and rearming for a planned attack on Japanese airfields on Formosa. The small force of B-17s operated against the Japanese invasion force until they were withdrawn to Darwin, in Australia's Northern Territory. In early 1942, the 7th Bombardment Group began arriving in Java with a mixed force of B-17s and LB-30/B-24s.A squadron of B-17 from this force detached to the Middle East to join the First Provisional Bombardment Group thus becoming the first American B-17 squadron to go to war against the Germans.[citation needed] After the defeat in Java, the 19th withdrew to Australia where it continued in combat until it was sent back home by General George C. Kenney when he arrived in Australia in mid-1942. In July 1942, the first USAAF B-17s were sent to England to join the Eighth Air Force. Later that year two groups moved to Algeria to join Twelfth Air Force for operations in North Africa. The B-17s were primarily involved in the daylight precision strategic bombing campaign against German targets ranging from U-boat pens, docks, warehouses and airfields to industrial targets such as aircraft factories.In the campaign against German aircraft forces in preparation for the invasion of France, B-17 and B-24 raids were directed against German aircraft production while their presence drew the Luftwaffe fighters into battle with Allied fighters.

Early models proved to be unsuitable for combat use over Europe and it was the B-17E that was first successfully used by the USAAF. The defense expected from bombers operating in close formation alone did not prove effective and the bombers needed fighter escorts to operate successfully.

During World War II, the B-17 equipped 32 overseas combat groups, inventory peaking in August 1944 at 4,574 USAAF aircraft worldwide. B-17s dropped 640,036 short tons (580,631 metric tons) of bombs on European targets (compared to 452,508 short tons (410,508 metric tons) dropped by the Liberator and 463,544 short tons (420,520 metric tons) dropped by all other U.S. aircraft).[clarification needed] The British heavy bombers, the Avro Lancaster and Handley Page Halifax, dropped 608,612 long tons (681,645 short tons) and 224,207 long tons (251,112 short tons) respectively.

 

Initial USAAF operations over Europe

 
"Combat boxes" of 12 B-17 during bombing missions.
 
Marks and letters on the tails of B-17 during WWII in Europe.

The air corps (renamed United States Army Air Forces (USAAF) on 20 June 1941), using the B-17 and other bombers, bombed from high altitudes using the then-secret Norden bombsight, known as the "Blue Ox",which was an optical electro-mechanical gyro-stabilized analog computer.The device was able to determine, from variables input by the bombardier, the point at which the aircraft's bombs should be released to hit the target. The bombardier essentially took over flight control of the aircraft during the bomb run, maintaining a level altitude during the final moments before release.

The USAAF began building up its air forces in Europe using B-17Es soon after entering the war. The first Eighth Air Force units arrived in High Wycombe, England, on 12 May 1942, to form the 97th Bomb Group.On 17 August 1942, 12 B-17Es of the 97th, with the lead aircraft piloted by Major Paul Tibbets and carrying Brigadier General Ira Eaker as an observer, were close escorted by four squadrons of RAF Spitfire IXs (and a further five squadrons of Spitfire Vs to cover the withdrawal) on the first USAAF heavy bomber raid over Europe, against the large railroad marshalling yardsat Rouen-Sotteville in France, while a further six aircraft flew a diversionary raid along the French coast.The operation, carried out in good visibility, was a success, with only minor damage unrelated to the enemy to one aircraft and half the bombs landing in the target area.The raid helped assuage British doubts about the capabilities of American heavy bombers in operations over Europe.

Two additional groups arrived in England at the same time, bringing with them the first B-17Fs, which would serve as the primary AAF heavy bomber fighting the Germans until September 1943. As the raids of the American bombing campaign grew in numbers and frequency, German interception efforts grew in strength (such as during the attempted bombing of Kiel on 13 June 1943), such that unescorted bombing missions came to be discouraged.

Combined offensive

The two different strategies of the American and British bomber commands were organized at the Casablanca Conference in January 1943. The resulting "Combined Bomber Offensive" would weaken the Wehrmacht, destroy German morale and establish air superiority through Operation Pointblank's destruction of German fighter strength in preparation for a ground offensive.The USAAF bombers would attack by day, with British operations – chiefly against industrial cities – by night.

 Boeing B-17F  radar bombing through clouds: Bremen,Germany, on13 November 1943.

Operation Pointblank opened with attacks on targets in Western Europe. General Ira C. Eaker and the Eighth Air Force placed highest priority on attacks on the German aircraft industry, especially fighter assembly plants, engine factories and ball-bearing manufacturers.[7] Attacks began in April 1943 on heavily fortified key industrial plants in Bremen and Recklinghausen.

Since the airfield bombings were not appreciably reducing German fighter strength, additional B-17 groups were formed, and Eaker ordered major missions deeper into Germany against important industrial targets. The 8th Air Force then targeted the ball-bearing factories in Schweinfurt, hoping to cripple the war effort there. The first raid on 17 August 1943 did not result in critical damage to the factories, with the 230 attacking B-17s being intercepted by an estimated 300 Luftwaffe fighters. The Germans shot down 36 aircraft with the loss of 200 men, and coupled with a raid earlier in the day against Regensburg, a total of 60 B-17s were lost that day.

A second attempt on Schweinfurt on 14 October 1943 would later come to be known as "Black Thursday".While the attack was successful at disrupting the entire works, severely curtailing work there for the remainder of the war, it was at an extreme cost.Of the 291 attacking Fortresses, 60 were shot down over Germany, five crashed on approach to Britain, and 12 more were scrapped due to damage – a total loss of 77 B-17s. A total of 122 bombers were damaged and needed repairs before their next flight. Out of 2,900 men in the crews, about 650 men did not return, although some survived as prisoners of war. Only 33 bombers landed without damage. These losses were a result of concentrated attacks by over 300 German fighters.

Such high losses of aircrews could not be sustained, and the USAAF, recognizing the vulnerability of heavy bombers to interceptors when operating alone, suspended daylight bomber raids deep into Germany until the development of an escort fighter that could protect the bombers all the way from the United Kingdom to Germany and back. At the same time, the German night fighting ability noticeably improved to counter the nighttime strikes, challenging the conventional faith in the cover of darkness.The Eighth Air Force alone lost 176 bombers in October 1943,[99] and was to suffer similar casualties on 11 January 1944 on missions to Oschersleben, Halberstadt and Brunswick. Lieutenant General James Doolittle, commander of the Eighth, had ordered the second Schweinfurt mission to be cancelled as the weather deteriorated, but the lead units had already entered hostile air space and continued with the mission. Most of the escorts turned back or missed the rendezvous, and as a result 60 B-17s were destroyed. A third raid on Schweinfurt on 24 February 1944 highlighted what came to be known as "Big Week",during which the bombing missions were directed against German aircraft production. German fighters would have to respond, and the North American P-51 Mustang and Republic P-47 Thunderbolt fighters (equipped with improved drop tanks to extend their range) accompanying the American heavies all the way to and from the targets would engage them. The escort fighters reduced the loss rate to below seven percent, with only 247 B-17s lost in 3,500 sorties while taking part in the Big Week raids.

By September 1944, 27 of the 42 bomb groups of the Eighth Air Force and six of the 21 groups of the Fifteenth Air Force used B-17s. Losses to flakcontinued to take a high toll of heavy bombers through 1944 but the war in Europe was being won by the allies and by 27 April 1945 (two days after the last heavy bombing mission in Europe), the rate of aircraft loss was so low that replacement aircraft were no longer arriving and the number of bombers per bomb group was reduced. The Combined Bomber Offensive was effectively complete.

Bomber defense

 
Part of a USAAF stream of over 1,000 B-17s

Before the advent of long-range fighter escorts, B-17s had only their .50 caliber M2 Browning machine guns to rely on for defense during the bombing runs over Europe. As the war intensified, Boeing used feedback from aircrews to improve each new variant with increased armament and armor. The number of defensive guns increased from four 0.50 in (12.7 mm) machine guns and one 0.30 in (7.62 mm) nose machine gun in the B-17C, to thirteen 0.50 in (12.7 mm) machine guns in the B-17G. But because the bombers could not maneuver when attacked by fighters, and needed to be flown straight and level during their final bomb run, individual aircraft struggled to fend off a direct attack.

 
German training model on how to attack a "flying porcupine" (fliegendes Stachelschwein)

A 1943 survey by the USAAF found that over half the bombers shot down by the Germans had left the protection of the main formation. To address this problem, the United States developed the bomb-group formation, which evolved into the staggered combat box formation where all the B-17s could safely cover any others in their formation with their machine guns, making a formation of the bombers a dangerous target to engage by enemy fighters. Luftwaffe fighter pilots likened attacking a B-17 combat box formation to encountering a fliegendes Stachelschwein,

"flying porcupine", with dozens of machine guns on a combat box formation of bombers, aimed at them from almost every direction. However, the use of this rigid formation meant that individual aircraft could not engage in evasive maneuvers: they had to fly constantly in a straight line, which made them vulnerable to the German flak. Moreover, German fighter aircraft later used the tactic of high-speed strafing passes rather than engaging with individual aircraft to inflict damage with minimum risk.

As a result, the B-17s' loss rate was up to 25% on some early missions (60 of 291 B-17s were lost in combat on the second Raid on Schweinfurt), and it was not until the advent of long-range fighter escorts (particularly the North American P-51 Mustang) resulting in the degradation of theLuftwaffe as an effective interceptor force between February and June 1944, that the B-17 became strategically potent.

 
Formation flying through dense flak over Merseburg, Germany

The B-17 was noted for its ability to absorb battle damage, still reach its target and bring its crew home safely. Wally Hoffman, a B-17 pilot with the Eighth Air Force during World War II, said, "The plane can be cut and slashed almost to pieces by enemy fire and bring its crew home."[128] Martin Caidin reported one instance in which a B-17 suffered a midair collision with a Focke-Wulf Fw 190, losing an engine and suffering serious damage to both the starboard horizontal stabilizer and the vertical stabilizer, and being knocked out of formation by the impact. The B-17 was reported as shot down by observers, but it survived and brought its crew home without injury. Its toughness was compensation for its shorter range and lighter bomb load compared to the B-24 and British Avro Lancaster heavy bombers.[clarification needed] Stories abound of B-17s returning to base with tails having been destroyed, with only a single engine functioning or even with large portions of wings having been damaged by flak. This durability, together with the large operational numbers in the Eighth Air Force and the fame achieved by the Memphis Belle, made the B-17 a key bomber aircraft of the war. Other factors such as combat effectiveness and political issues also contributed to the B-17's success.

The B-17 design went through eight major changes over the course of its production, culminating in the B-17G, differing from its immediate predecessor by the addition of a bombardier-operated, remotely controlled chin turret with two .50 in (12.7 mm) caliber M2 Browning machine guns under the nose, also fitted to the final eighty-six production aircraft of the B-17F-DLs built by Douglas, after being first tried in May 1943 as part of the increased armament system of the YB-40. This eliminated the B-17's main defensive weakness in head-on attacks.

Luftwaffe attacks

 
The severely damaged All Americancontinues to fly after collision with an attacking Bf 109 fighter, eventually landing without crew injuries.

After examining wrecked B-17s and B-24s, Luftwaffe officers discovered that on average it took about 20 hits with 20 mm shells fired from the rear to bring them down.Pilots of average ability hit the bombers with only about two percent of the rounds they fired, so to obtain 20 hits, the average pilot had to fire one thousand 20 mm (0.79 in) rounds at a bomber. Early versions of the Fw 190, one of the best German interceptor fighters, were equipped with two 20 mm (0.79 in) MG FF cannons, which carried only 500 rounds when belt-fed (normally using 60-round drum magazines in earlier installations), and later with the better Mauser MG 151/20 cannons, which had a longer effective range than the MG FF weapon. Later versions carried four or even six MG 151/20 cannon and twin 13 mm machine guns. The German fighters found that when attacking from the front, where fewer defensive guns were mounted (and where the pilot was exposed and not protected by armor as he was from the rear), it took only four or five hits to bring a bomber down. To rectify the Fw 190's shortcomings, the number of cannons fitted was doubled to four with a corresponding increase in the amount of ammunition carried, creating the Sturmbock bomber destroyer version. This type replaced the vulnerable twin-engine Zerstörerheavy fighters which could not survive interception by P-51 Mustangs flying well ahead of the combat boxes in an air supremacy role starting very early in 1944 — as ordered by the Eighth Air Force's commander, then-Maj. Gen. Jimmy Doolittle — to clear any Luftwaffe defensive fighters from the skies. By 1944, a further upgrade to Rheinmetall-Borsig's 30 mm (1.2 in) MK 108 cannons mounted either in the wing, or in underwing, conformal mount gun pods, was made for theSturmbock Focke-Wulfs as either the /R2 or /R8 field modification kits, enabling aircraft to bring a bomber down with just a few hits.

 
B-17G-15-BO "Wee Willie", 322d BS, 91st BG, after direct flak hit on her 128th mission.

The adoption of the 21 cm Nebelwerfer-derived Werfer-Granate 21 (Wfr. Gr. 21) rocket mortar by the Luftwaffe in mid-August 1943 promised the introduction of a major "stand-off" style of offensive weapon – one strut-mounted tubular launcher was fixed under each wing panel on the Luftwaffe's single-engine fighters, and two under each wing panel of a few twin-engine Bf 110 daylight Zerstörer aircraft. However, due to the slow 715 mph velocity and characteristic ballistic drop of the fired rocket (despite the usual mounting of the launcher at about 15° upward orientation), and the small number of fighters fitted with the weapons, the Wfr. Gr. 21 never had a major effect on the combat box formations of Fortresses.

The Luftwaffe also fitted heavy-calibre Bordkanone-series 37, 50 and even 75 mm (2.95 in) cannon as anti-bomber weapons on twin-engine aircraft such as the specialJu 88P fighters, as well as one model of the Me 410 Hornisse but these measures did not have much effect on the American strategic bomber offensive. The Me 262, however, had moderate success against the B-17 late in the war. With its usual nose-mounted armament of four MK 108 cannons, and with some examples later equipped with the R4M rocket, launched from underwing racks, it could fire from outside the range of the bombers' .50 in (12.7 mm) defensive guns and bring an aircraft down with one hit, as both the MK 108's shells and the R4M's warheads were filled with the "shattering" force of the strongly brisant Hexogen military explosive.

Specifications (B-17G)

 
3-view projection of a B-17G, with inset detail showing the "Cheyenne tail" and some major differences with other B-17 variants

Data from The Encyclopedia of World Aircraft

General characteristics

Performance

Armament

  • Guns: 13 × .50 in (12.7 mm) M2 Browning machine guns in 8 positions (2 in the Bendix chin turret, 2 on nose cheeks, 2 staggered waist guns, 2 in upper Sperry turret, 2 in Sperry ball turret in belly, 2 in the tail and one firing upwards from radio compartment behind bomb bay)
  • Bombs:
    • Short range missions (<400 mi): 8,000 lb (3,600 kg)
    • Long range missions (≈800 mi): 4,500 lb (2,000 kg)
    • Overload: 17,600 lb (7,800 kg)