The Pratt & Whitney F100 (company designation JTF22) is a low bypass afterburning turbofan engine. It was designed and manufactured by Pratt & Whitney to power the U.S. Air Force's "FX" initiative in 1965, which became the F-15 Eagle. The engine was to be developed in tandem with the F401 which shares a similar core but with an upscaled fan for the U.S. Navy's F-14 Tomcat. The F401 was later abandoned due to costs and reliability issues. The F100 also powered the F-16 Fighting Falcon for the Air Force's Lightweight Fighter (LWF) program.

In 1967, the United States Navy and United States Air Force issued a joint engine Request for Proposals (RFP) for the F-14 Tomcat and the FX, which became the parallel fighter design competition that led to the F-15 Eagle in 1970. This engine program was called the IEDP (Initial Engine Development Program) and was funded and managed out of the Aeronautical Systems Division (ASD) at Wright-Patterson AFB. Under ASD, a Systems Project Office Cadre was assigned to manage both the FX Aircraft and Engine definition phase. The Turbine Engine Division of the Air Force Propulsion Laboratory was employed in a support role to assist ASD Systems Engineering in evaluations of technical risks. Later upon selection of the F-15 the ASD engineering cadre became the F-15 Systems Project Office.

The IEDP was created to be a competitive engine design/demonstration phase followed with a down select to one winning engine design and development program. General Electric and Pratt & Whitney were placed on contract for an approximately 18-month program with goals to improve thrust and reduce weight to achieve a thrust-to-weight ratio of 8. At the end of the IEDP, General Electric and Pratt & Whitney submitted proposals for their engine candidates for the aircraft that had been selected in the FX Competition, the McDonnell Douglas F-15. The Pratt & Whitney proposal was selected as the winner and the engine was designated the F100. The Air Force would award Pratt & Whitney a contract in 1970 to develop and produce F100-PW-100 (USAF) and F401-PW-400 (USN) engines. The Navy would use the engine in the planned F-14B and the XFV-12 project but would cut back and later cancel its order after the latter's failure due to costs and reliability issues, and chose to continue to use the Pratt & Whitney TF30 engine from the F-111 in its F-14s.

The F100 is a twin spool, axial flow, afterburning turbofan engine. It has a 3-stage fan driven by a two-stage low-pressure turbine and a 10-stage compressor driven by a two-stage high-pressure turbine. The initial F100-PW-100 variant generates nearly 24,000 lbf (107 kN) of thrust in full afterburner and weighs approximately 3,000 lb (1,361 kg), achieving its target thrust-to-weight ratio of 8 and providing the F-15 with its desired thrust-to-weight ratio of greater than 1:1 at combat weight.

The F100-PW-100 first flew in an F-15 Eagle in 1972 with a maximum continuous power rating of 12,410 lbf (55.2 kN), military power of 14,690 lbf (65.3 kN), and afterburning thrust of 23,930 lbf (106.4 kN) with 5-minute limit. Due to the advanced nature of engine stemming from ambitious performance goals, numerous problems were encountered in its early days of service including high wear, stalling and "hard" afterburner starts. These "hard" starts could be caused by failure of the afterburner to start or by extinguishing after start, in either case the large jets of jet fuel were lit by the engine exhaust resulting in high pressure waves causing the engine to stall; these stagnation stalls usually occurred at high Mach and high altitude, and could seriously damage the turbine if the condition was not corrected. The problems were contributed by pilots making much more abrupt throttle changes than previous fighters and engines due to the excess thrust available. Early problems were eventually solved by the development of the F100-PW-220 in the early 1980s, which the -100 could be upgraded to.

The F-16 Fighting Falcon entered service with the F100-PW-200; compared to the -100, the -200 had some additional redundancies for single-engine reliability as well as almost identical thrust ratings. In particular, a "proximate splitter" — an extension of the internal casing behind the fan that splits the core and bypass airflow — was introduced on the -200 that reduced the severity of the high pressure waves from "hard" afterburner starts. This greatly reduced the rate of stagnation stalls, and the -200 on the F-16 saw much better reliability than the -100 on the F-15, although some of the issues from the -100 remained. Similarly, these problems were eventually solved by the F100-220, which the -200 could be upgraded to as well.

Data from DTIC, Smithsonian National Air and Space Museum

Due to the unsatisfactory reliability, maintenance costs, and service life of the F100-100/200, Pratt & Whitney embarked on an extensive upgrade program in order to address these issues. The Air Force also began funding the General Electric F101 Derivative Fighter Engine, which eventually became the F110, as a competitor to the F100 to coerce more urgency from Pratt & Whitney. The result of Pratt & Whitney's improvement efforts was the F100-PW-220, which eliminates almost all stall-stagnations and augmentor instability issues from the -100 as well as doubling time between depot overhauls. Reliability and maintenance costs were also drastically improved, and the engine incorporates a digital electronic engine control (DEEC). The -220 engine produces static thrust of 14,590 lbf (64.9 kN) in military (intermediate) power and 23,770 lbf (105.7 kN) afterburning, very slightly lower than the static thrust of the -100/200, but the -220 has better dynamic thrust across most of the envelope.