"B2431"
The U.S. built and flew a few disc aircraft during WW2. The Chance Vought
V-173
first flew 23 November 1942, which had excellent STOL capabilities and was
apparently impossible to stall.

AVRO Canada experimented with a flying saucer type design in the 1950s.
This was funded in part by the USAF. It was known as the Avrocar. It was
abandoned as I guess technology had not been developed (i.e. computer flight
control systems etc) for controlling successful flight. Do a search with
'google' for "AVRO Flying Saucer" if you want to read more about this
project.
Ed Majden