Flight testing of the X-4 Bantam began in 1948, the design having been inspired by German WWII research and developments, with both examples constructed being passed to the NACA (National Advisory Committee for Aeronautics) in 1950. The first X-4 was grounded after only 10 flights and the second (seen here) was used in the joint USAF/NACA program which ended in 1953. Instability at high speed led to the conclusion that semi-tailless aircraft were not suitable for transonic flight with the technology then available. It now hangs in the fascinating Research & Development Gallery

Transonic tailless twinjet research aircraft. Flew 1949-53.