Boeing XB-15 - Design and Development

Design and Development

The specification that produced the XB-15 began in mid-1933 as "Project A", USAAC discussions regarding the possibility of flying a very large bomber with a range of 5,000 mi (8,000 km). In April 1934 the USAAC contracted with Boeing and Martin to design a bomber capable of carrying 2,000 lb (910 kg) at 200 mph (320 km/h) over a distance of 5,000 miles. Boeing gave the project the internal name of Model 294, while the USAAC called it the XB-15. Martin's design, the XB-16, was judged inferior by the USAAC before a prototype was built, and was canceled.

The Boeing design team, headed by Jack Klystra, initially intended the aircraft to use 2,600 hp (1,900 kW) Allison V-3420 liquid-cooled inline engines; since these were not ready, 850 hp (630 kW) Pratt & Whitney R-1830 air-cooled radial engines were used instead.

Starting in August 1934, Boeing began designing the Model 299 in answer to a proposal by the USAAC to replace the Martin B-10 bomber. The Model 299 design team incorporated elements of the Model 294, especially the use of four engines, and the Boeing 247. The Model 299 design team worked alongside Klystra's team, but difficulties in fabricating such a large aircraft slowed progress on the 294. The Model 299 flew first, on 28 July 1935.

In mid-1935, the USAAC combined Project A with Project D; a proposal asking for "the maximum feasible range into the future." The combined program was designated BLR for "Bomber, Long Range". The XB-15 was renamed the XBLR-1; it was joined under the BLR program by two other projects: one from Douglas Aircraft, the XBLR-2 which later became the XB-19; and one from Sikorsky Aircraft called the XBLR-3, later canceled. The next year, the XBLR was dropped and the Boeing prototype was once again the XB-15.

Unusual features that the XB-15 pioneered included an autopilot, deicing equipment, and two gasoline generators used as auxiliary power units independent of the main engines to power the 110-volt electrical system. The engines were serviceable in flight using an access tunnel inside the wing. The aircraft contained a sizable crew compartment with bunkbeds, a galley and a lavatory. Finally, in September 1937 construction was finished, and on 15 October it first flew. Its double-wheel main landing gear remained down from takeoff to landing. On 2 December 1937, the XB-15 flew from Seattle to Wright Field in Ohio to be accepted by the USAAC for testing.

With the weaker engines installed, the specified speed of 200 mph was not quite reached even when the aircraft was empty; the best speed attained in level flight was 197 mph (317 km/h). Loaded with the specified 2,000 pounds, the maximum speed was a disappointing 145 mph (233 km/h). This was considered too slow for a combat aircraft, and the project was abandoned. However, Boeing engineers projected that the prototype would be capable of carrying the heaviest air cargo to date: a load of 8,000 lb (3,600 kg).

The design challenges stemming from the great size of the XB-15 were difficult to master, but the lessons learned by Boeing were later applied to the Model 314 flying boat. In 1938, the USAAC proposed to update the XB-15 to make the slightly larger Y1B-20, using more powerful engines, but the Secretary of War, Harry Hines Woodring, canceled the project before construction began, in favor of the expensive Douglas XB-19. Boeing went ahead with an internal redesign of the XB-15 called Model 316, a very heavy bomber with a high wing, a pressurized cabin and tricycle gear. The Model 316 was not built. The progression of design work starting with the XB-15 finally bore fruit with the Model 345 presented to the USAAC in May 1940; the very heavy bomber which resulted in the Boeing B-29 Superfortress.