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... but unfortunately they never made it past the prototype phase.

The National Archives has released a declassified document about "USAF Project 1794" which was a 1956 design study by Canadian aicraft company Avro Aircraft on a saucer design intended to reach 100,000 feet with a top speed of Mach 4 and a range of 1000 nautical miles, with vertical takeoff and landing capability.

Considering the highest flying, fastest aircraft the USAF has put into service, the SR71 Blackbird, couldn'y hit those specs (except for range, but it's a bit irrelevant with refueling), that's a pretty aggresive concept, saucer shape notwithstanding.  The project was canceled without successful demonstration ... as far as we know.

Apparently they couldn't successfully reverse engineer the wreckage from Roswell, or something.

The archives have some cutaway drawings at the link.