Information about the proposed craft have been around for years, but the declassified documents include diagrams that clearly demonstrate the scope of the government's "project." The documents were published by the U.S. National Archives, with news reaching the media on Monday.
The project even had a code name, Project 1794. Developed by the U.S. Air Force and Avro Canada in the 1950s, one declassified memo says that Project 1794 "is a flying saucer capable of between Mach 3 and Mach 4” (approximately 2,300 - 3,000 MPH, depending on altitude and temperature), a service ceiling of over 100,000 feet, and a range of around 1,000 nautical miles (1,150 miles).
It's unclear if all that information was theoretical, or if prototypes were actually built. Avro Canada, which as noted above teamed with the USAF on Project 1794, also worked on the VZ-9 Avrocar. That vehicle was built, but was not designed with the lofty expectations of Project 1794.
Basically the same as Project 1794 - but a lot smaller - the VZ-9 had initial specifications of a maximum speed of 300 MPH with a service ceiling of 10,000 feet. In practice (video embedded), it never achieved a height of greater than three feet or a speed of greater than 35 MPH.