The F-111 began in the early 1960s as the TFX, an ambitious project to combine the USAF requirement for a fighter-bomber with the United States Navy's need for a long-range air defence fighter to replace the F-4 Phantom II and the F-8 Crusader. The prevailing fighter design philosophy of the day was to concentrate on very high speed, raw power, and air-to-air guided missiles. (This would change within a few years as experience with then-modern fighter types like the Phantom showed that close-in dogfighting remained a important part of air combat, leading to the reintroduction of guns as well as missiles and a new emphasis on manoeuvrability, but not until well after the basic F-111 design was completed.)
For the US Navy, the trend to ever bigger, more powerful fighters posed a problem: the current generation of naval fighters were already barely capable of landing on an aircraft carrier deck; and a still larger and faster fighter would be more difficult again. An airframe optimised for high speed (most obviously with a high-angle wing sweep) is inefficient at cruising speeds, which reduces range, payload and endurance, and leads to very high landing speeds. On the other hand, an airframe with a straight or modestly swept wing, while easier to handle and able to carry heavy loads a long way on a minimum of fuel, has lower ultimate performance for combat. It was these considerations that led to the famous F-111 variable geometry, the 'swing-wing'.
The birth of the TFX was marked by controversy, with the Air Force, the Navy and the US Government all pulling the project in different directions. At one stage, it was even planned to use it for the United States Army and the United States Marine Corps as a close support aircraft! Several manufacturers submitted bids; the final two shortlisted were General Dynamics and Boeing. The USAF and the USN, in one of the few matters they were able to agree on, both wanted the Boeing design, but United States Secretary of Defense Robert McNamara overruled them and chose the General Dynamics aircraft instead, citing cost issues—an extraordinary irony considering the eventual price of F-111s!
The design eventually emerged as a 20 tonne aircraft (empty) with a maximum takeoff weight of almost 50 tonnes, powered by two afterburning Pratt & Whitney TF-30 turbofans in the 80 kN class, with side-by-side accommodation for a crew of two. The high mounted wings were attached to a pair of giant swivels, allowing it to take off, land, and loiter with a modest 16 degree sweep (for maximum lift and minimum landing speed), cruise at high sub-sonic speeds with a 35 degree sweep, or rotate the wings right aft to a 72.5 degree sweep for a very fast maximum speed of Mach 2.4—particularly so for a bomber, which the F-111 had become by this time, its "F" (for "fighter") designation notwithstanding.
Production versions of the F-111 did not have ejection seats. The pressurized crew compartment ejected as a self-contained survival module and descended under a 70 foot parachute.
First flight was in December 1964 and entry into service with the USAF began in 1967. It was the first variable geometry aircraft. Despite its clear advantages, variable geometry remains a relatively unusual feature in military aircraft, due to higher cost, and the extra weight imposed by the swing-wing mechanism. Nevertheless, several other types have followed, including the Soviet Sukhoi Su-17 (1966), MiG-23 (1967) and Tupolev Tu-160 bomber (1981), the US F-14 Tomcat naval fighter (1970) and B-1 bomber (1974), and the European Panavia Tornado (1974).