Boeing B-17 Flying Fortress – Introduction and Development

The Boeing B-17 Flying Fortress is one of the most famous aircraft of the Second World War. It earned that fame with the Eighth Air Force, carrying out daylight bombing raids over Hitler’s Fortress Europe. It first flew in 1935, before work had even begun on any of the famous British four engined heavy bombers of the war. Over the next ten years a total of 12,731 B-17s were built, and the B-17 served in every major theatre of the Second World War.

Model 247

The B-17 contained a number of features that first appeared in the Boeing Model 247. This was an all-metal streamlined transport plane, with low wings, retractable landing gear, and supercharged Pratt and Whitney Wasp engines. The shape of the fuselage was clearly an influence on the XB-15 and B-17 bombers that followed.

XB-15

Just before Boeing began work on the B-17 they had been approached to work on a massive strategic bomber, the XB-15. The resulting aircraft was nearly 50 feet wider and 20 feet longer than the B-17. Although work on the two projects began within months of each other, the first flight of the XB-15 was two years after than of the first B-17 prototype. The XB-15 also owned a great deal to the Model 247, with a similar fuselage and tail plane, but it had rather different wings, with the same broadly triangular shape as the B-17. 

Model 299

The prototype for the B-17 was developed as the Boeing Model 299, in response to an Army Air Corps requirement for a coastal defence bomber. In 1933 a proposal for a design competition for a multi-engined bomber was distributed amongst the aircraft manufactures. The official specification for the new design was issued in May 1934. It called for a multi-engined aircraft with a top speed of 200-250mph at 10,000 feet, a ceiling of 20,000-25,000 feet, a cruising speed of 170-220mph and an endurance of six to ten hours at cruise speed.

Boeing B-17G Flying Fortress

Boeing’s main competitor came from the Martin B-10B and Douglas B-18A. These were both twin engined designs, as had been every previous American bomber. Boeing decided to produce a small four engined design, using the extra power provided by the four engines to increase the performance rather than the size of the aircraft. Despite this the Model 299 was the largest landplane in America when it took to the air.

The Model 299 was an all-metal aluminium aircraft, powered by four Pratt & Whitney R-1690 Hornet engines, providing 750hp each. The Model 299 was designed to carry a crew of eight. It was armed with five machine guns, one in a rounded Perspex nose that could rotate around 360 degrees to allow the machine gun bubble to be moved and four in gun pods around the fuselage (one dorsal, one ventral and two in the waist).

The Model 299 made its first flight on 28 July 1935. By then it had already been given the name “Flying Fortress”, by a journalist who was covering the rollout ceremony (so much for secrecy!). On 20 August 1935 the aircraft was flown from Boeing’s factory at Seattle to Wright Field, Ohio, where it was to undergo official testing. It completed this journey of 2,100 miles in just over nine hours, at an average speed of 233mph, proving it fulfilled the speed and range requirements of the original specification.

Boeing Model 299 from the left

The Model 299 was by far the best aircraft entered into the competition, but it was also the most expensive to produce. As a four engined design it had to overcome a certain amount of scepticism, which was not reduced when the aircraft was destroyed in a crash. In order to prevent the large control surfaces of the Model 299 being damaged in high winds, it was possible to lock them in place when on the ground. On 30 October the aircraft was being tested by Major Ployer Hill of the Army Air Corps and Boeing’s Chief Test Pilot, Leslie Turner. The control locks were left on, and the aircraft crashed soon after take off, killing both men.

Before the crash the Air Corps had already recommended the purchase of 65 B-17s. One of the arguments used against the aircraft was that those 65 aircraft were to replace 185 aircraft already authorized for the 1936 financial year. It was not at all clear that 65 four engined B-17s would be a better anti-shipping weapon than nearly three times that many smaller aircraft. It is worth remembering that during the Second World War level bombing was not an effective anti-shipping weapon. A moving ship at sea was a very small target from the sort of altitudes that these attacks would have to be made from, especially when the target was taking evasive action. Official Air Corps doctrine for attacking ships with B-17s required twenty bombers to attack each ship in formation, which it was believed would produce a 7% hit rate. 185 dive bombers would have been rather more effective.

The Air Corps’s real interest in the Model 299 was as a strategic bomber. As a result of the crash the size of the order was dramatically scaled down to include thirteen fight test aircraft and one static test bed. This contract was signed on 17 January 1936, allowing work to begin on the YB-17. The original production contract was won by the Douglas B-18.

YB-17/ Y1B-17

The thirteen service test aircraft were ordered as the YB-17. This designation was later changed to the Y1B-17 to indicate the fund being used to develop the aircraft. The crew of the aircraft was reduced from eight to six, with the result that not all of the guns could be fired at the same time. The engine was changed from the Pratt and Whitney engines of the Model 299 to Wright R-1820-39 Cyclone engines. These would be used in every future version of the aircraft. The first YB-17 first flew on 2 December 1936, and was delivered to the Army in January 1937. All thirteen aircraft were complete by August 1937. The test program did not run smoothly. The Cyclone engine was not yet as reliable as it would later become, while the brakes were also prone to overheating. One of the thirteen aircraft was badly damaged in a crash on 7 December 1936, further delaying the mass production of the design. A determined publicity campaign was mounted by the Air Corps to convince the public that the B-17 was an impressive aircraft, and not an overcomplicated 

Y1B-17A

One airframe had been purchased for use as a static test bed, to be tested to destruction. One of the Y1B-17s then flew through a severe storm during normal testing without suffering any damage, removing the need for the destruction tests, and allowing the airframe to be used in the flight tests. This aircraft was used to test the turbo-supercharger, which used the exhaust gases from the engines to force air into the combustion chamber of the engine at high altitudes, improving the performance of the aircraft.

The first flight of the Y1B-17 was made on 29 April 1938, with the supercharger above the engines. This interfered with the airflow over the wing, and so Boeing relocated the turbo-supercharger under the engine nacelle. The modified aircraft flew on 20 November 1938. Tests early in 1939 confirmed the benefits of the turbo-supercharger. The ceiling of the aircraft increased by 7,000 feet, to 38,000 feet, and its top speed rose from 239mph at 5,000 feet to 271mph at 25,000 feet. All future B-17s would carry turbo-superchargers. On 3 August 1937, even before these tests had been carried out, Boeing had received a contract for ten B-17Bs, the first of over 12,000 production aircraft

The B-17 was produced in large numbers by Boeing, Douglas and Vega. By the end of the war Boeing was scaling down production to concentrate on the B-29.

B-17 production to August 1945.

Boeing B-17 Flying Fortress, Frederick A. Johnsen. A well researched and illustrated history of the B-17, with a very strong section on its combat record, an interesting chapter on the efforts made to improve the aircraft (including a number of suggestions that didn't enter production) and a good selection of colour pictures of the aircraft. [see more]

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