The Development of the B-17.

The history of the B17 began in 1934 when the US Army Air Corps issued a specification for a new multi engined bomber. The specifications called for a bomber that would have a top speed of 200-250 mph at 10,000 feet, a cruising speed of 170-200 mph at the same altitude, a range of 6 to 10 hours (1700miles), and a service ceiling of 20,000-25,000 feet.

Boeing responded by designing a four engined aircraft - the model 299. This was at a time when twin engine designs were regarded as large aircraft and as a result Boeing's four engine concept was greeted with scepticism by some within the Army and government. Many felt that such a large aircraft would be too complicated and unwieldy in combat and openly voiced their criticism of such a wild proposition.

The designers at Boeing however, firmly believed that the future for bomber aircraft lay in a design that would not only carry a respectable payload a long distance, but one that could fly high, fast and defend itself against fighter interceptors. To achieve this with a twin engined design was then impossible even using the most powerful engines available. In 1934, the most powerful American aero engines were developing around 1000hp with anticipated development increasing this to around 1400hp.

Studies were underway into producing bigger engines, however the major manufacturers of the time, mainly Pratt and Whitney, Wright and the Allison companies indicated that reliable production versions with powers exceeding 1400hp were three to five years away.

Accordingly Boeing designed its Model 299 around one of the most powerful production engines that was then available, the Pratt and Whitney Hornet, a nine cylinder radial developing 750hp. One factor in the choice of a radial engine over an in-line engine such as the Allison V-12, was that it was a proven design with growth potential, and being air cooled offered a simpler installation than liquid cooled engines. Another significant factor was that similar radial engines were also being manufactured by the Wright company and it made sense not to be locked into one supplier.

The final design was a low-wing monoplane that combined aerodynamic features of the XB-15, a proposed giant bomber capable of reaching Europe that was still in the design phase and the Boeing Model 247 civil transport.

Prototype Model 299 on rollout

The Model 299 first flew in July 1935 and immediatley demonstrated its superiority over other contract contenders. In competitive flight tests it soon set records for speed and distance, outpacing all army interceptors then in service. However, an unexpected accident with the prototype almost ended the 299's career. During one test flight on October 30, 1935, the pilots forgot to remove the elevator gust locks during their preflight inspection. Gust locks were then a new concept, being needed to lock the aircraft's large control surfaces in place while on the ground to prevent wind damage. The result was the aircraft crashed on take off, destroying the aircraft, killing both pilots and injuring the test crew.

Critics of the Boeing 299 immediately siezed this opportunity to point out that they were right about it being too complicated for military service. The contract went instead to the Douglas company for their adaptation of their twin engined DC-2 airliner, the B-18. Fortunately for Boeing there were senior officers within the army who realised that the cause of the crash was nothing more than simple pilot error and impressed by its superior performance, they managed to extend the contract requirements so that new specifications were drawn up that fitted the Boeing design.

Boeing was stretched financially, having put most of their resources into the 299's development, and it took plenty of clever persuasion to have their financiers agree to extend funding to complete a second prototype. The next prototype was soon in the air and again set about demonstrating its superiority over any bomber then flying. It was at this stage when one journalist, having seen the unprecedented defensive armament of five fifty calibre machine guns, commented on it as being a "Flying fortress" . The name stuck and is now part of history.

An order was placed by the Army Air Corps and limited run of test models, now named the YB-17 began. The YB-17 had five machine guns, a bomb load of 4,800 pounds and a crew of nine.

Y1B-17 over New York

A new model, the Y1B-17, was built switching to more powerful powerful Wright Cyclone engines of 900hp. Twelve were delivered to the USAAC's 2nd Bombardment Group for trials in June 1937. One of these was soon equipped with new Moss/General Electric turbochargers that later became standard on all B17s. The turbochargers raised the aircraft's ceiling to over 25,000ft and gave it a top speed of 300mph. This made it the fastest and highest flying bomber of the day, with a performance that was unequalled by most fighter aircraft then in service.

For the next two years, the US Army Air Corps conducted extensive testing using these pre-production models, determining various configurations, operational methods, bombing trials etc. There were many problems to work out, as high altitude long range bombing was a new concept and much had to be learned before a commitment to a final design could be made. It was during this period that the B-17 was mated with the Norden bombsight - developed by immigrant Carl Norden in 1932. (see our coming story about the Norden sight)

The first production order was for 39 B-17Bs with turbo-charged engines placed in June 1939, and while these were in production another order for the B-17C was placed. This had seven machine guns instead of the original five, better armor and self sealing tanks.

The next milestone in the aircraft's career came in 1940 when the Royal Air Force, desperate for aircraft in the war against Germany, ordered 20 B-17Bs for combat evaluation.