The McDonnell XP-67 "Moonbat" was impressive.
Impressive in terms of the innovative airframe design, and particularly impressive in terms of how problematic it was.
Intended to serve as a high-speed, long-range, high-altitude interceptor, the designers of the XP-67 spent significant time and effort on the airframe design.
In an attempt to reduce interference drag, the wing, fuselage, and engine nacelles were smoothly blended together.
With the exception of the tail, sharp angles between airframe surfaces were completely eliminated.
While this was anticipated to result in impressive speed and performance, the airplane was cursed by engine problems.
The engines, experimental Continental XI-1430s, were liquid-cooled inverted V-12s that ate bearings, overheated, produced insufficient power, and caught fire
And although the fuselage design was indeed innovative, the flight characteristics were nearly as problematic as the engines.
The aircraft's low-speed handling was so erratic and unpredictable, test pilots declined to test its spin characteristics for fear the spin would be unrecoverable.
In the end, the sole XL-67 succumbed to an engine fire that began in flight and then spread to the fuselage after a successful emergency landing.