mustang 63

The Boeing XB-38 flying fortress - a single B-17E with Allison V-1710 V type engines rather than the standard Wright R-1820. Only one prototype was built, and it made its first flight on May 19, 1943. It had a much higher top speed, but lower service ceiling - and the engine manifold joints leaked exhaust gauses after the first few flights. 

On the ninth flight, June 16, 1943, the third engine caught fire – forcing the crew to bail out. 

The prototype was destroyed and the project was canceled. The V-1710 engines were needed for other planes, like the P-38 Lightning, P-39 Airacobra, P-40 Warhawk, P-51A model Mustang, and P-63 Kingcobra.