The first D-558-II Skyrocket, Bureau No. 37973 and NACA 143, began flight testing on February 4, 1948 with Douglas pilot John Martin. Initially a ground-launched turbojet aircraft, it was converted in 1954-1955 for air launch with rocket power. It made 123 flights, all but one in the Douglas contractor program; John McKay flew its sole NACA rocket-powered familiarization flight on September 17, 1956.