Let the clues begin...
According to one site it only flew once (for about two minutes) and when it abruptly landed there was a heated/furious discussion between the designer and the test pilot, while he was still seated in the cockpit. It never flew again. Estimated top speed was to be around 320 mph but the prototype was under-powered.
Not unique, but the engine installation was novel for the time. The flight occurred in 1938.