Declassified documents reveal that the Air Force worked stealthily on a flying saucer craft in the 1950s. From Sebastian Anthony at Extreme Tech:

“The aircraft, which had the code name Project 1794, was developed by the USAF and Avro Canada in the 1950s. One declassified memo, which seems to be the conclusion of initial research and prototyping, says that Project 1794 is a flying saucer capable of ;between Mach 3 and Mach 4,’ (2,300-3,000 mph) a service ceiling of over 100,000 feet (30,500m), and a range of around 1,000 nautical miles (1,150mi, 1850km).

As far as we can tell, the supersonic flying saucer would propel itself by rotating an outer disk at very high speed, taking advantage of the Coandă effect. Maneuvering would be accomplished by using small shutters on the edge of the disc (similar to ailerons on a winged aircraft). Power would be provided by jet turbines. According to the cutaway diagrams, the entire thing would even be capable of vertical takeoff and landing (VTOL).”