Bell P-39 Airacobra
The Airacobra was an innovative and promising mid-engine design that was completely screwed up by the Army Engineering and Procurement Department. Somebody at Wright Field was apparently obsessed with aerodynamic drag, and in a fit of streamlining ended up removing a lot of really useful parts of the airplane. They forced Bell to make design changes that turned the P-39 into a troublesome, spin-prone aircraft which soon earned the nickname the "Iron Dog".
Cool stuff for Airacrobra lovers: