When it first took to the air the Boeing XB-15 was the biggest aircraft in the world, but it had already been superseded by the smaller but more efficient B-17 Flying Fortress. Both aircraft had been developed at around the same time, but the B-17 first flew in July 1935, while the XB-15 did not take to the air until August 1937, just over two years later. The two aircraft represented the extremes of four engined aircraft design – the XB-15 was as large as possible while the B-17 was the smaller four engined bomber yet built.
Through the 1930s many aviation experts believed that the bomber would always get through. In the early 1930s this idea was at least partly justified. Modern bombers of the period could reach 200mph, and could outpace the fighter aircraft of the day. With no radar to provide advance warning of an enemy attack, an interceptor aircraft would need to be significantly faster than the bombers it was attacked, and no such aircraft yet existed. There was also a belief that fighters fast enough to intercept 200mph bombers would be too difficult to be “efficiently or safely operated either individually or in mass” (General Westover).
This theory would later be disproved. The invention of radar gave the defenders a chance to direct their fighters into the path of an oncoming bomber force, while improvements in fighter design produced aircraft like the Supermarine Spitfire or Messerschmitt Bf 109, each capable of over 350mph (without posing a threat to their pilots).
Work on the design that would produce the XB-15 began in July 1933 with a study at the Material Division at Wright Field. It had been decided that a long range bomber would need to be able to carry a 2,000lb bomb load. The purpose of the Material Division study was to decide how far and how fast that bomb load could be carried. The study concluded that it would be possible to build an aircraft that could carry a 2,000lb bomb load form 5,000 miles and 200mph.
In December 1933 this specification was then submitted to the War Department as Project A and received tentative approval. On 12 February 1933 a budget of $609,300 was approved in principle, and on 12 May the Chief of Staff of the Army gave the Air Corps permission to begin contract negotiations with Boeing and Martin. The Army Arc Corps specifications called for an aircraft capable of “the destruction by bombs of distant land or naval targets” and the ability “to reinforce Hawaii, Panama and Alaska without the use of intermediate servicing facilities”.
Engineering work on the aircraft began in January 1934. Flight testing began on 15 August 1937 and it was delivered to the Army Air Corps in March 1938.
The resulting aircraft closely resembles a scaled up version of the early B-17 prototypes. The main visual difference was the thickness of the massive wings, which were so large that they could contain a passageway to the engines. Six guns (three 0.30in and three 0.50in were carried in six separate positions – three around the rear fuselage, one on the nose, one in a chin turret and one in a turret above the centre of the wings. Internally the aircraft was very well appointed, with sleeping quarters, a kitchenette and a toilet.
Its great size and weight was the downfall of the XB-15. Compared to the B-17 it was slower, with a lower service ceiling and a small bomb load but with the same amount of engine power. Its only advantage was a much longer range. By the time the XB-15 took to the air, Boeing had already received an order for 39 B-17Bs.
The single XB-15 was converted to act as a cargo plane, with the designation XC-105. In this role it broke a number of recordings, one of which saw it carry a 31,167.6lb payload to a height of 8,200 feet. It remained in service until 1945, when it was finally dismantled.
Engines: Pratt & Whitney R-1830-11
Horsepower: 4x850hp, total 3,400hp
Maximum Weight: 70,706lbs
Max Speed: 197mph
Cruising Speed: 171mph
Length: 87ft 7in
Bomb load: 3,500 miles
Armament: Three 0.30in and three 0.50on machine guns in six positions