Design work started in 1955 by Stanley Hiller Jr and Hiller Aircraft Corporation received a manufacturing contract and funding from the U.S. Air Force to build the only X-18 ever produced. The 33,000 lb loaded X-18 underwent extensive ground tests beginning in December 1958, and made its first conventional flight on 24 November 1959. It made partial conversions with wing angles of up to 33Â°. On the 20th flight it had a propeller pitch control problem at 10,000 ft and went into a spin. Thankfully the crew managed to land safely, just. But after this lucky escape the X-18 was grounded. Four years later it was scrapped.