The D-21 was first launched from an M-21 on 5 March 1966. The drone was released but stayed close to the M-21's back for a few seconds, which seemed like "two hours" to the M-21 crew. A second launch took place on 27 April 1966; the D-21 reached its operational altitude of and speed of over , though it was lost due to a hydraulic pump failure after a flight of over . The USAF's interest in the program continued and more D-21s were ordered after the second launch. A third flight took place on 16 June with the D-21 flying through its complete flight profile, though its camera hatch was not released due to an electronics failure.