Early requirements The May
1960 U-2 incident, in which an American
CIA U-2 reconnaissance plane was shot down over the USSR, stunned the United States government. Besides greatly damaging US–Soviet relations, the incident showed that the Soviet Union had developed a
surface-to-air missile that could reach aircraft above 60,000 ft (18,000 m). Consequently, the USAF
Strategic Air Command (SAC) and the
RAF Bomber Command's plans to send subsonic, high-altitude
Boeing B-47 Stratojet and
V bomber formations into the USSR were realized to be much less viable. By 1960, SAC had begun moving to
low-level penetration, which greatly reduced
radar detection distances. At the time, SAMs were ineffective against low-flying aircraft, while
interceptor aircraft had less of a speed advantage at low altitudes. The USAF's
Tactical Air Command (TAC) was largely concerned with the
fighter-bomber and deep strike/
interdiction roles. TAC was in the process of receiving its latest design, the
Republic Thunderchief, which was designed to deliver nuclear weapons fast and far, but required long runways. A simpler
variable-geometry wing configuration with the pivot points farther out from the aircraft's centerline was reported by
NASA in 1958, which made swing-wings viable. This led USAF leaders to encourage its use. In June 1960, the USAF issued specification
SOR 183 for a long-range, interdiction/strike aircraft able to penetrate
Soviet air defenses at very low altitudes and high speeds. Specifically, it was to be capable of at least of low-level flight, half of which was to be at a speed of no less than Mach 1.2. Furthermore, the specification also called for the aircraft to possess
short takeoff-and-landing (STOL) capabilities to permit operations from short, unprepared airstrips that had a length of no more than . Around this time, the
United States Navy had been seeking a long-range, high-endurance,
fleet air defense fighter to protect its
carrier battle groups against long-range
antiship missiles launched from Soviet jet bombers and submarines. It would need a more powerful radar and longer-range missiles than the
F-4 Phantom II it would replace. The Navy had proposed a
subsonic, straight-winged aircraft, the
Douglas F6D Missileer in the late 1950s. The Missileer could carry six long-range missiles and loiter for five hours, but would be defenseless after firing its missiles. NASA's simplification made the variable-geometry wings practical. Variable geometry offered high speeds, and maneuverability with heavier
payloads, long range, and STOL capability.
Tactical Fighter Experimental The USAF and Navy were both seeking new aircraft when
Robert McNamara was appointed
secretary of defense in January 1961. Both sought high-supersonic, twin-engined, two-seat aircraft that could carry heavy armament and fuel loads and probably use variable-geometry wings. On 14 February 1961, McNamara formally directed the services to study the development of a single aircraft that would satisfy both requirements. Early studies indicated that the best option was to base the design on the USAF requirement, and use a modified version for the Navy. In June 1961, Secretary McNamara ordered the go-ahead of
Tactical Fighter Experimental (TFX), despite USAF and Navy efforts to keep their programs separate. According to aviation author Peter E. Davis, military officials were disconcerted by McNamara's focus on compromised requirements for financial reasons. The two services could agree only on swing-wing, two-seat, twin-engined design features. The USAF wanted a
tandem-seat aircraft for low-level penetration ground attack, while the Navy wanted a shorter, high-altitude interceptor with
side-by-side seating to allow the pilot and
radar operator to share the radar display. Also, the USAF wanted the aircraft designed for 7.33 g with Mach 2.5 speed at altitude and Mach 1.2 speed at low level with an approximate length of . The Navy had less strenuous requirements: 6 g with Mach 2 speed at altitude and high subsonic speed (about Mach 0.9) at low level with a length of . The Navy also wanted the aircraft with a nose large enough for a diameter radar dish. McNamara developed a basic set of requirements for TFX based largely on the USAF's requirements, and on 1 September 1961, ordered the USAF to develop it. In December, proposals were received from
Boeing,
General Dynamics,
Lockheed,
McDonnell,
North American, and
Republic. The evaluation group found all the proposals lacking, but Boeing and General Dynamics were selected to submit enhanced designs. Boeing's
proposal was recommended by the selection board in January 1962, with the exception of the engine, which was not considered acceptable. The board also directed alterations to radar and missile storage and a switch from
ejection seats to a crew-escape capsule. Both companies provided updated proposals in April 1962. USAF reviewers favored Boeing's offering, while the Navy found both submissions unacceptable for its operations. A congressional investigation into the procurement process was conducted, but did not change the selection. On 1 May 1964, the definitized contract was issued for the program, including flight testing, spares, ground equipment, training devices, static and fatigue test data, and the production of an initial 23 F-111 aircraft; it was structured as a
fixed-price incentive-fee (FPIF) contract with a ceiling price of $529 million, along with provisions for deficiency correction among other operational clauses and performance criteria.
Design phase General Dynamics' design team was led by
Robert H. Widmer. Recognizing its lack of experience with carrier-based fighters, General Dynamics teamed with
Grumman in November 1963 for the assembly and testing of the F-111B. In addition, Grumman would also build the aft fuselage and the landing gear of the F-111A. The General Dynamics and Grumman team faced ambitious requirements for range, weapons load, and aircraft weight. Thus, the F-111 was designed to incorporate numerous features that were new to production military aircraft, such as variable-geometry wings and afterburning turbofan engines. This use of unfamiliar features has been attributed as a major cause of the aircraft's protracted development and weight increases. The F-111A and F-111B shared the same airframe structural components and
Pratt & Whitney TF30-P-1 turbofan engines. They featured side-by-side crew seating in an escape capsule as required by the Navy. The
F-111B's nose was shorter as the aircraft could fit on existing carrier elevator decks, and had wingtips to improve on-station endurance time; it also carried an
AN/AWG-9 pulse-Doppler radar to guide its
AIM-54 Phoenix missiles. The USAF's F-111A would be equipped with the AN/APQ-113 attack radar and the
AN/APQ-110 terrain-following radar and air-to-ground armament. During September 1963, the F-111A mockup was inspected. Early flights of the F-111, which included supersonic flights, demonstrated favorably simplistic maintenance requirements, among other qualities. Various changes to the program were enacted throughout 1965; this was chiefly in response to a steep climb in unit costs from $4.5 million to $6 million. On 10 May 1967, a new, multiyear FPIP contract replaced the prior procurement process, increasing the total aircraft on order to 493 F-111s of multiple models, including 23 F-111Bs intended for the US Navy, 24 F-111Cs for the
Royal Australian Air Force, and 50 F-111Ks intended for the
Royal Air Force. Early flights of the F-111 were troubled by compressor surges and stalls across certain portions of the flight regimen. General Dynamics had elected to use an uncommon, spike-shaped, variable intake for the engine for the performance. During February 1965, the F-111A achieved a speed of Mach 1.3 while flying with an interim intake design. Separately, cracks in the F-111's
wing attachment points were first discovered in 1968 during ground fatigue testing; during the following year, the crash of an F-111 was attributed to a cracked wingbox. The resolution involved the redesigning of the attachment structure and necessitated testing to ensure adequate design and workmanship. On 31 July 1970, the grounding was lifted. Category I flight testing of the F-111A, which had started in 1964, continued through to 31 March 1972. Australia would procure its own model, the F-111C. Subsequently, the improved F-111E, F-111D, and F-111F models were developed for the USAF. The strategic bomber FB-111A and the EF-111 electronic-warfare versions were later developed for the USAF. Production of the F-111 ended in 1976, following the completion of 563 aircraft. ==Design==