Aware of the competition from the
Rolls-Royce Conway turbofan, Pratt & Whitney decided to develop the JT3D turbofan from the JT3C
turbojet for later deliveries of the Boeing 707 and the Douglas DC-8, then nearing entry into service. A 2-stage fan replaced the first 3 stages of the 9-stage JT3C LP compressor. On the LP turbine, the second stage was enlarged and a third stage added. Unlike GE with the CJ805-23, Pratt & Whitney had not undertaken any transonic fan research prior to designing the JT3D, so they were unable to incorporate a single stage unit into the specification. Instead P&W designed a 2-stage unit based on some research they had done to support the J91 nuclear turbojet. On the Boeing 707 the JT3D fan nacelle was relatively short, whereas the Douglas DC-8 installation had a full-length fan cowl. Pratt & Whitney provided a kit whereby JT3Cs could be converted to the JT3D specification, and performance, during an overhaul. In 1959, important orders for the engine were the
Boeing 707-120B and
Boeing 720B when
American Airlines ordered one 707 powered by JT3D turbofans and
KLM ordered a JT3D-powered
Douglas DC-8. Earlier 707s and DC-8s had been powered by the JT3C and
JT4A turbojets, and the improved efficiency of the turbofan soon attracted the airlines. A JT3D-powered 707-123B and 720-023B (the suffix B was to indicate a turbofan-powered aircraft) entered service with American Airlines on the same day, March 12, 1961. The
Boeing KC-135 Stratotankers were all originally powered by turbojet engines. With the demise of many airline 707s, the
United States Air Force took the opportunity to buy the surplus airframes and use the engines to re-fit the KC-135As used by the Air National Guard and reserve squadrons with the civilian JT3D (designated TF33-PW-102). Over 150 aircraft were modified and the former KC-135A was re-designated the KC-135E. After long service for both airlines and air forces, the number of JT3D-powered aircraft is steadily decreasing. One hundred thirty five KC-135s use the JT3D, while 354 were fitted with
CFM International CFM56 engines, which provide greater thrust, lower fuel consumption, and increased operational flexibility due to their lower noise footprint. The noise of the JT3D is one of the reasons
NATO has debated re-fitting their
E-3 Sentry AWACS fleet, since the aircraft are subject to restrictions that aircraft with modern engines are not. Operational flexibility would be further increased due to the ability of higher power engines to increase the ceiling of the aircraft, extending the horizon for radar surveillance; for instance,
RAF,
French and
Saudi E-3s routinely fly higher than NATO/USAF counterparts. In 1961, the TF33-powered
Boeing B-52H Stratofortress entered service. The "H" model of the B-52 was the only production variant of the heavy bomber to be fitted with turbofan engines, and is the only model remaining in United States Air Force service. It is expected to remain as a mainstay of the Air Force
heavy bomber fleet until at least 2040, with options for replacing the 8 TF33 engines with more modern equivalents being considered. In April 2020, the USAF released a request for proposals for 608 commercial replacement engines, with the plan to award the contract in May 2021. In September 2021, the USAF announced that the TF33 would be replaced by the
Rolls-Royce F130. ==Variants==