The United States Air Force opened the "CX-X Program" in 1964, intending to produce a next-generation strategic airlifter. Of the several airframe and engine proposals returned for consideration, Lockheed's aircraft and General Electric's engine were selected for the new design in 1965.
The high-bypass turbofan was a huge leap in engine performance, offering a thrust of 43,000 pounds, while improving fuel efficiency by about 25%. The TF39 had an 8-to-1 bypass ratio, 25-to-1 compressor pressure ratio, a 2,500 °F (1,370 °C) turbine temperature made possible by advanced forced-air cooling. The first engine went for testing in 1965. Between 1968 and 1971, 463 TF39-1 and -1A engines were produced and delivered to power the C-5A fleet.
Mechanically, the TF39 is rather unusual for a high bypass ratio turbofan; the single stage is snubbered ('Snubbers' are protuberances that stick-out at right angles to the fan aerofoil somewhere between mid-span and blade tip). The snubbers on adjacent fan blades butt-up against each other, in a peripheral sense and improve the vibration characteristics of the blade to the fan rotor which has a set of inlet guide vanes for the outer bypass section and the core booster stage located in front of the fan rotor, rather than behind. This unique design is clearly seen from the front.