The VZ-9 Avrocar is marketed as a “what if” kit, but to my surprise there actually WERE two of these things built and “flown”.
The idea originated in the early 1950s as a U.S. Air Force project, built by AVRO Canada, for a high-speed, high performance fighter with high speed and maneuverability. The performance didn’t pan out, and the U.S. Army took over the project with the idea of having something like a high speed, highly maneuverable helicopter. The two prototypes were built in 1958 and 59. The project was abandoned in 1961.
One of the prototypes is on display at the US Air Force Museum at Wright-Patterson AFB, Dayton, Ohio.