Boeing XB-15

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 . 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. 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 Boeing 247 and the Model 294, especially its use of four engines. The Model 299 design team worked alongside Klystra's team, but difficulties in fabricating such a large aircraft slowed progress on the 294. 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 designation was dropped and the Boeing prototype was once again the XB-15. However, Boeing engineers projected that the prototype would be capable of carrying the heaviest air cargo to date: a load of . 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, which essentially used the XB-15's wing design with four of the more powerful Wright Twin Cyclone fourteen-cylinder radials for power. In 1938, the USAAC proposed to update the XB-15 to make the slightly larger Y1B-20, again using four Wright Twin Cyclones as with the Boeing 314, 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 USAAF's Boeing B-29 Superfortress. ==Operational history==

agent and Major Caleb V. Haynes supervise the loading of emergency medical supplies on the XB-15 in early 1939 The single prototype was assigned to the 2nd Bombardment Group at Langley Field, Virginia. Following the 24 January 1939 Chillán earthquake in Chile, the prototype flew a relief mission, carrying medical supplies. Commanded by Major Caleb V. Haynes, the aircraft carried of American Red Cross emergency supplies to Santiago, making only two stops along the way, at France Field in the Panama Canal Zone, and at Lima, Peru. Haynes was awarded certificates issued by the National Aeronautics Association (NAA) for an international record for "the greatest payload carried to an altitude of 2,000 meters". The XB-15 was not fast for a bomber but it was the fastest aircraft that could carry so much weight, and for such distances. In July 1939, Haynes received certificates from the NAA for an international speed record with a payload. The latter performance also established a national closed circuit distance record of . It displayed nose art depicting an elephant carrying a large crate on its back labeled "supplies". ==Operators==

; • United States Army Air Corps : 2d Bombardment Group • United States Army Air Forces : 20th Troop Carrier Squadron ==Specifications (XB-15)==