American Fisher P-75 Eagle
At the outset of the war, the United States Army Air Corps was in the process of developing a number of prototype aircraft, but it would be a few years before models like the very capable P-51 Mustang were ready. Meantime, the Air Corps had to make do with fighters and interceptors that were inferior to German and Japanese planes.
One such plane under development was the GM/Fisher P-75 Eagle. The Air Corps wanted a plane that was fast and well-armed. The Allison engine company developed a new model called the V-3420, a massive 24-cylinder engine that combined a pair of V-1710 engines into one. The Fisher Body Division of General Motors set to work building an aircraft around the engine.
Fisher used preexisting parts of other successful planes to design and build the P-75 including the Dauntless dive bomber, P-51, and P-40. The large engine was situated in the middle of the aircraft and powered twin contra-rotating propellers.
But piecing together a new design from existing planes was not a successful model. The P-75 was slow and sluggish in its role as an interceptor, so the Army Air Corps passed on it. Fisher attempted to market the plane instead as a long-range bomber escort but by then better designs had already been fielded.