The engine's first test run took place in a test rig in December 1966 at East Hartford, Connecticut, with the engine's first flight in June 1968 mounted on a Boeing B-52E which served as a 747 testbed.
O primeiro encendido tivo lugar en decembro de 1966 en East Hartford, Connecticut, e o motor voou por vez primeira en xuño de 1968 montado nun Boeing B-52E que servía como banco de probas para o 747.