Boeing B-17 Flying Fortress XB-38

Lockheed's Vega division modified the ninth B-17E built to test liquid-cooled Allison V-1710-89 inline engines, in case the Wright R-1820 radial became scarce. It first flew on 19 May 1943; it showed a somewhat higher top speed but had to be grounded after only a few flights because of exhaust-gas leaks at the engine manifold joints. Once that was fixed, the ninth flight (16 June 1943) ended when the inboard starboard engine caught fire; the crew bailed out and the aircraft was lost, ending the project. The performance gains were minor and Allison engines were considered more urgently needed for fighters.