Pratt & Whitney Aircraft began development of its first production turbofan, the JT3D, in the late 1950s, by taking advantage of the service-proven JT3 (J57) gas generator, and adding a two-stage front fan. The JT3D made its first flight on a Boeing 707-120 on June 22, 1960, and powered the Boeing 707-120B, Boeing 720, and Douglas DC-8-50 airliners.

Although initially funded by the U.S. Air Force as the T33, the JT3D is an historic commercial jet transport engine. It was Pratt & Whitney's first commercial production turbofan, and fended off competition from the Rolls Royce Conway, which had a lower bypass ratio. It only required a simple modification to existing JT3 engines, which could be carried out by operators using a manufacturer provided kit. The conversion had a marginal effect on weight, but increased thrust by 35 percent, reduced fuel consumption by 15 to 22 percent, and cut takeoff noise by 10 dB.