THE US army was planning to build a supersonic 'flying saucer' in the 1950s, newly declassified documents reveal.

Documents from the US Air Force Aeronautical Systems were recently published by the National Declassification Centre revealing the secret plans known as 'Project 1794'.

The cover of a 'Final Development Summary Report' from 1956 shows an illustration of a flying saucer of the kind made popular in science fiction movies from the 1950s.

A Canadian company, Avro Aircraft Ltd, had been commissioned to construct a prototype of the disc-shaped craft.

It was designed to take off vertically and would have a top speed of Mach 4, a maximum altitude of 30km and a range of over 1000 nautical miles.

Technical schematics reveals one version of the craft which would only have been big enough for a single pilot, and another with a bigger cockpit.

The document concluded that the project was "feasible and the aircraft can be designed to have satisfactory handling through the whole flight range from ground cushion take-off to supersonic flight at very high altitude".

It estimated that it would cost another $US3 million to take the project to a prototype, two years later.

However as a subsequent video reveals, the prototype literally almost failed to get off the ground, and the project was eventually cancelled in 1961.

Originally published as US secret flying saucer plan revealed