A flying saucer made for the U.S. military, devised in the Cold War (period 1958-1959) by Canadian Scientists, was a completely top secret project. Avro Canada, a Canadian aircraft manufacturing company, designed and built two prototypes of this aircraft, under the funding of the U.S. military that believed on an “alien” looking plane. At those times as a common story on media, the UFO cases were arising, and news speculated some UFOs were actually Soviet-built saucer-shaped vehicles.
Some airliners turn into military aircraft as well. This is the An-12 “Cub”, a military aircraft that had a version of a passenger airplane in the Soviet Union. Constructed simultaneously with the An-10 “the airliner” but with structural differences to allow military operations, one of them is a defensive tail gun. This airplane was Designed in terms of capability and comparison to the famous C-130 “Hercules”. It is surprising but this plane has many variants still in service as civil operations as military around the world.