Boeing YB-40 Flying Fortress Bomber Escort Prototype Aircraft
The YB-40 Flying Fortress was a wartime conversion of the classic B-17 bomber to fulfill a flying gunship role - 25 were built.
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Like other classic aircraft of the World War 2 period (1939-1945), the Boeing B-17 "Flying Fortress" heavy bomber was subject to many experiments and conversion projects during its time in service. The YB-40 was developed as one of the former and intended to showcased the B-17 as a sort of flying "gun bus" to defend bomber formations to-and-from enemy targets. The project was not an outright success but did yield some twenty-five examples before the end and influence several key changes of the B-17 line going forward.
Prior to the availability of long-range fighter escorts like the North American P-51 Mustang and Republic P-47 Thunderbolt, thought was given to outfitting bomber airframes with additional machine guns, suitable ammunition stocks, and improved armor protection to help them serve as formation escorts. These aircraft were designed to accompany large American bomber formations over enemy airspace, provide point defense against intercepting enemy fighters, and return with the formation once the war load was dropped on enemy targets below. The B-17 seemed a good a starting point as any for it was a proven player and available in the numbers required.
Project V-139 marked the first example of such a creation and this involved a production-quality B-17F model. Lockheed (Vega) handled the early conversion work which involved addition of machine guns at various key locations and armor protection at the gunner's positions. The single guns at each open-air beam position (the openings staggered for better gunner movement in the heat-of-battle) was doubled and a second dorsal turret was added aft of the first. A twin-gunned Bendix powered turret was installed at the chin position. The bombing equipment and bomb bay were deleted and, in the latter's place, a reserve for additional stocks of ammunition was added. The Sperry ball turret under the belly was retained and the cheek machine guns, at the sides of the forward fuselage, were still in play (though not initially installed as part of the conversion work).