B17 Flying Fortress

The Boeing B17 Flying Fortress is an American four engine heavy bomber aircraft developed for the U.S. Army Air Corps (USAAC). Competing against Douglas and Martin for a contract to build 200 bombers, the Boeing entry outperformed both the other competitors and more than met the Air Corps' expectations. Although Boeing lost the contract due to the prototype's crash, the Air Corps was so impressed with Boeing's design that they ordered 13 B17's. The B17 Flying Fortress went on to enter full scale production and was considered the first truly mass-produced large aircraft, eventually evolving through numerous design advancements, from B17A to G.

The B17 was primarily employed in the daylight precision strategic bombing campaign of World War II against German industrial and civilian targets. The United States Eighth Air Force based in England and the Fifteenth Air Force based in Italy complemented the RAF Bomber Command's nighttime area bombing in Operation Pointblank, to help secure air superiority over the cities, factories and battlefields of Western Europe in preparation for Operation Overlord. The B17 also participated, to a lesser extent, in the War in the Pacific.

From its pre war inception, the USAAC touted the aircraft as a strategic weapon; it was a potent, high flying, long ranging bomber capable of unleashing great destruction yet able to defend itself. With the ability to return home despite extensive battle damage, its durability, especially in belly landings and ditching, quickly took on mythic proportions. Stories and photos of B17s surviving battle damage widely circulated, boosting its iconic status. Despite an inferior range and bomb load compared to the more numerous B24 Liberator, a survey of Eighth Air Force crews showed a much higher rate of satisfaction in the B17. With a service ceiling greater than any of its Allied contemporaries, the B17 established itself as a superb weapons system, dropping more bombs than any other U.S. aircraft in World War II. Of the 1.5 million tonnes of bombs dropped on Germany, 500,000 were dropped from B17's.

On 8th August 1934, the U.S. Army Air Corps (USAAC) tendered a proposal for a multi engined bomber to replace the Martin B10. Requirements were that it would carry a "useful bomb load" at an altitude of 10,000 ft (3 km) 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 miles (3200 km) and a speed of 250 mph (400 km/h). The Air Corps were looking for a bomber capable of reinforcing the air forces in Hawaii, Panama, and Alaska. The competition would be decided by a "fly off" at Wright Field in Dayton, Ohio. Boeing competed with the Douglas DB1 and Martin Model 146 for the Air Corps contract.

The prototype B17, designated Model 299, was designed by a team of engineers led by E. Gifford Emery and Edward Curtis Wells and built at Boeing's own expense. It combined features of the experimental Boeing XB15 bomber with the Boeing 247 transport airplane. The B17 was armed with bombs (up to 4,800 pounds (2200 kg) on two racks in the bomb bay behind the cockpit) and five 0.30 inch (8 mm) caliber machine guns, and was powered by Pratt & Whitney R1690 radial engines each producing 750 horsepower (600 kW) at 7,000 ft (2100 m).

The first flight of the Model 299 was on 28th July 1935, with Boeing chief test pilot Les 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. Boeing was quick to see the value of the name and had it trademarked for use. On 20th August, the prototype flew from Seattle to Wright Field in nine hours and three minutes at an average speed of 235 mph (378 km/h), much faster than the competition.

At the fly off, the four engine Boeing design displayed superior performance over the twin engine DB1 and Model 146, and General Frank Maxwell Andrews of the GHQ Air Force believed that the long range capabilities of four engine large aircraft were more efficient than shorter-ranged twin engined airplanes. His opinions were shared by the Air Corps procurement officers and, even before the competition was finished, they suggested buying 65 B17's.

Development continued on the Boeing Model 299, and on 30th October 1935, the Army Air Corps test pilots Ployer, Hill, and Tower, took the Model 299 on a second evaluation flight. The crew forgot to disengage the airplane's "gust lock," a device that held the bomber's movable control surfaces in place while the plane was parked on the ground, and having taken off, the aircraft entered a steep climb, stalled, nosed over and crashed, killing two. The crashed Model 299 could not finish the evaluation, and while the Air Corps was still enthusiastic about the aircraft's potential, Army officials were daunted by the much greater expense per aircraft. Army Chief of Staff Malin Craig cancelled the order for 67 B17's, and ordered 133 of the twin engine Douglas B18 Bolo instead.

Regardless, the USAAC had been impressed by the prototype's performance and, on 17th January 1936, the Air Corps ordered, through a legal loophole, 13 YB17's for service testing. The YB17 incorporated a number of significant changes from the Model 299, including more powerful Wright R1820/39 Cyclone engines replacing the original Pratt & Whitneys.

On 1st March 1937, 12 of the 13 YB17's were delivered to the 2nd Bombardment Group at Langley Field in Virginia, and used to help develop heavy bomber techniques and work out other bugs. One suggestion was the use of a checklist, to avoid accidents such as the Model 299's. In one of their first missions, three B17's, following lead navigator Lt. Curtis LeMay, were sent by General Andrews to "intercept" the Italian ocean liner Rex 800 statute miles (1300 km) off the Atlantic coast and take photographs. The successful mission was widely publicised.

The 13th YB17 was delivered to the Material Division at Wright Field, Ohio, to be used for flight testing.

A 14th YB17 (37/369), originally constructed for ground testing of the airframe's strength, was upgraded and fitted with exhaust-driven turbochargers. Scheduled to fly in 1937, it encountered problems with the turbochargers and its first flight was delayed until 29th April 1938. Modifications cost Boeing US$100,000 and took until spring 1939 to complete, but resulted in an increased service ceiling and maximum speed. The aircraft was delivered to the Army on 31st January 1939 and was re-designated B17A to signify the first operational variant.

In late 1937, the Air Corps ordered ten more aircraft, designated B17B and, soon after, another 29. Improved with larger flaps, rudder and Plexiglas nose, the B17B's were delivered between July 1939 and March 1940. They equipped two bombardment groups, one on each U.S. coast.

Prior to the attack on Pearl Harbor, fewer than 200 B17's were in service with the Army, but production quickly accelerated, and the B17 became the first truly mass produced large aircraft. The aircraft went on to serve in every World War II combat zone, and by the time production ended in May 1945, 12,731 aircraft had been built by Boeing, Douglas and Vega (a subsidiary of Lockheed).

The B17 began operations in World War II with the RAF in 1941, USAAF Eighth Air Force and Fifteenth Air Force units in 1942, and was primarily involved in the daylight precision strategic bombing campaign against German industrial targets. Operation Pointblank guided attacks in preparation for a ground assault.

During World War II, the B17 equipped 32 overseas combat groups, inventory peaking in August 1944 at 4,574 USAAF aircraft worldwide, and dropped 640,036 long tons (650,195 tonnes) of bombs on European targets (compared to 452,508 tons (451,691 tonnes) dropped by the Liberator and 463,544 tons (420,520 tonnes) dropped by all other U.S. aircraft). Approximately 4,750, or one third, of B17s built were lost in combat.

The Royal Air Force (RAF) entered World War II with no heavy bomber of its own and while by 1941, the Short Stirling and Handley Page Halifax had become its primary bombers, in early 1940, the RAF entered into an agreement with the U.S. Army Air Corps to be provided with 20 B17C's, redesignated Fortress I. Their first operation was against Wilhelmshaven on 8 July 1941. At the time, the Air Corps considered high altitude flight to be 20,000 ft (6 km) but, to avoid being intercepted by fighter aircraft; the RAF bombed the naval barracks from 30,000 ft (9 km). They were unable to hit their targets and temperatures were so low that the machine guns froze up. On 24th July, they tried another target, Brest in France, but again missed completely.

By September, after the RAF had lost eight B17C's in combat or to accidents, Bomber Command had abandoned daylight bombing raids due to the Fortress I's poor performance. The remaining aircraft were transferred to different commands for deployment to various duties including coastal defence. The experience had showed both the RAF and USAAF that the B17C was not ready for combat, and that improved defenses, larger bomb loads and more accurate bombing methods were required, which would be incorporated in later versions. Moreover, even with these improvements, it was the USAAF and not the RAF that was willing to remain faithful to using the B17 as a "day" bomber.

The RAF transferred its remaining Fortress I aircraft to Coastal Command for use as very long range patrol aircraft. These were later augmented in August 1942 by 19 Fortress Mk II and 45 Fortress Mk IIA (B17F and B17E, respectively - the USAAF offered the B17F before offering the B17E, thus the apparently reversed designations). A Fortress from No. 206 Squadron RAF sank U627 on 27th October 1942: the first of 11 U boat kills credited to RAF Fortress bombers during the war.

The Air Corps (renamed United States Army Air Force or USAAF in 1941), utilising the B17 and other bombers, bombed from high altitudes using the then secret Norden Bombsight, which was an optical electro mechanical gyro stabilized computer. During daylight bombing missions and sorties, the device was able to determine, from variables input by the bombardier, the point in space at which the bomber's ordnance type should be released to hit the target. The bombardier essentially took over flight control of the aircraft during the bomb run, maintaining a level attitude during the final moments.

The USAAF began building up its air forces in Europe using B17E's soon after entering the war. The first Eighth Air Force units arrived in High Wycombe, England on 12th May 1942, to form the 97th Bomb Group. On 17th August 1942, 18 B17E's of the 97th, including Yankee Doodle, flown by Major Paul Tibbets and Brigadier General Ira Eaker, were escorted by RAF Spitfires on the first USAAF raid over Europe, against railroad marshalling yards at Rouen Sotteville in France. The operation was a success, with only minor damage to two aircraft.

The two different strategies of the American and British Bomber commands were organized at the Casablanca Conference in January 1943. The resulting Operation Pointblank described a "Combined Bomber Offensive" that would weaken the Wehrmacht and establish air superiority in preparation of a ground offensive.

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.

On 17th April 1943, an attack on the Focke Wulf plant at Bremen by 115 Fortresses met with little success. Sixteen aircraft were shot down, and 48 others were damaged. The attacks did succeed, however, in diverting about half the Luftwaffe's fighter force to anti bomber operations.

Since the airfield bombings were not appreciably reducing German fighter strength, as additional B17 groups were formed, 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 17th August 1943 did not result in critical damage to the factories, with the 230 attacking B17's being intercepted by an estimated 300 Luftwaffe fighters. Thirty six aircraft were shot down with the loss of 200 men, and coupled with a raid earlier in the day against Regensburg, a total of 60 B17's were lost that day.

A second attempt on 14th October 1943 would later come to be known as "Black Thursday". Of the 291 attacking Fortresses, 59 were shot down over Germany, one ditched in the English Channel, five crashed in England, and 12 more were scrapped due to battle damage or crash-landings (more by AA guns than the Luftwaffe), a total loss of 77 B17's. One hundred and twenty two bombers were damaged to some degree 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 POW's. Five were killed and 43 wounded in the damaged aircraft that made it home, and 594 were listed as Missing in Action. Only 33 bombers landed without damage. These losses of air crews could not be sustained, and the USAAF, recognising the vulnerability of heavy bombers against interceptors, 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. The Eighth Air Force alone lost 176 bombers in October 1943. The Eighth Air Force was to suffer similar casualties on 11th January 1944 on missions to Oschersleben, Halberstadt and Brunswick. Doolittle had ordered the 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, as a result 60 B7's were destroyed. A third raid on Schweinfurt on 24 February 1944 highlighted what came to be known as "Big Week". With P51 Mustang and P47 Thunderbolt fighters (equipped with improved drop tanks to extend their range) escorting the American heavies all the way to and from the targets, only 11 of 231 B17's were lost. The escort fighters reduced the loss rate to below seven percent, with only 247 B17's lost in 3500 sorties while taking part in the Big Week raids.

By September 1944, 27 of the 40 bomb groups of the Eighth Air Force and six of the 21 groups of the Fifteenth Air Force utilized B17's. Losses to flak continued to take a high toll of heavy bombers through 1944, but 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.

Charactaristics

Crew: 10: Pilot, co-pilot, navigator, bombardier/nose gunner, flight engineer-top turret gunner, radio operator, waist gunners (2), ball turret gunner, tail gunner
Length: 74 ft 4 in (22.66 m)
Wingspan: 103 ft 9 in (31.62 m)
Height: 19 ft 1 in (5.82 m)
Wing area: 1,420 ft² (131.92 m²)
Airfoil: NACA 0018 / NACA 0010
Empty weight: 36,135 lb (16,391 kg)
Loaded weight: 54,000 lb (24,495 kg)
Max takeoff weight: 65,500 lb (29710 kg)
Powerplant: 4× Wright R-1820-97 "Cyclone" turbosupercharged radial engines, 1,200 hp (895 kW) each

Performance

Maximum speed: 287 mph (249 knots, 462 km/h)
Cruise speed: 182 mph (158 knots, 293 km/h)
Range: 1,738 nmi (2,000 mi, 3,219 km) with 2,722 kg (6,000 lb) bombload
Service ceiling 35,600 ft (10,850 m)
Rate of climb: 900 ft/min (4.6 m/s)
Wing loading: 38.0 lb/ft² (185.7 kg/m²)
Power/mass: 0.089 hp/lb (150 W/kg)