Key point: While excessively fast, it wasn't suited for the 1950s security environment.
Before the advent of ICBMs in the 1950s, the horsemen of the nuclear apocalypse would have trampled Washington and Moscow in the form of high-altitude bombers.
Existing U.S. fighters such as the F-86 Sabre were seen as too slow to meet this threat. So in 1949, the U.S. Air Force put out a request for a high-altitude supersonic interceptor that could intercept and destroy high-flying Soviet nuclear bombers before they dropped their loads.
Designated the 1954 Interceptor project to mark the year it was to enter service, the Air Force received nine proposals, of which three were chosen for preliminary development: Convair with a design that later became the F-102 Delta Dagger, Lockheed with a plane that later became the F-104 Starfighter, and Republic Aircraft with the AP-57, later renamed the XF-103.
Of the three designs, the XF-103 was the most advanced. Republic proposed an aircraft that could fly 2,600 miles per hour—faster than three times the speed of sound—to an altitude of 80,000 feet. For the early 1950s, when subsonic F-86s and MiG-15s were dogfighting over Korea at speeds of a torpid few hundred miles per hour, the XF-103 would have seemed more rocket than airplane.
Perhaps that's because the F-103 was almost a rocket. Even a drawing of the aircraft shows what looks something like a cruise missile. To achieve such high speeds, Republic—which later designed the F-105 Thunderchief of Vietnam fame—envisioned a dual propulsion system. A Wright XJ-67 turbojet engine would have powered the XF-103 during takeoff and normal flight.
But for that extra burst of speed to catch the incoming herd of Soviet Badger, Bear and Bison bombers, the XF-103 would have been equipped with a ramjet engine. Ramjets basically gulp down air from the front of the aircraft, mix it with fuel, and then shoot the mixture out the back. It's a relatively simple system, with the drawback that an airplane or rocket must already be moving at faster than Mach 1 in order for the air to be compressed enough for the ramjet to swallow it. The XF-103's turbojet would propel the aircraft to sufficient speed for the ramjet to kick in.
The XF-103 was to be armed with a long-range radar, six GAR-3 Falcon infrared or radar-guided air-to-air missiles, plus thirty-six unguided Mighty Mouse 2.75-inch air-to-air rockets. Including the Mighty Mouse rockets was a good idea, because the Falcon—the Air Force's first operational air-to-air guided missile—was a total flop in Vietnam, with fifty-four missiles scoring only five hits. Though the XF-103 carried no cannon, the lack of which hampered U.S. fighters over Vietnam, lining up a cannon shot at faster than Mach 3 with 1950s fire control radar would have been difficult.
The XF-103 was to be equipped with a unique ejection system. If the cockpit were to lose pressure, a shield stowed under the seat would rise up, enclosing the pilot in a pressurized pod. The pilot could fly the aircraft back to base using basic flight controls and a periscope, or if ejection were necessary, the pod would be lowered on rails out of the bottom of the fuselage and then released.
However, the XF-103 would never actually progress beyond a ground mock-up. “It was clear that the XF-103 was so far ahead of the state of the art that it was much too risky to be a serious contender for the 1954 Interceptor project,” explains aviation writer Joe Baugher. This made the competing Convair F-102 for all practical purposes the winner of the contest, and the Air Force began to lose interest in the XF-103. Continuing delays and cost overruns caused the program later to be cut back to only one prototype. The Wright XJ67 engine encountered even more delays and ultimately never materialized. Plans to substitute the Wright J65 turbojet proved to be unrealistic. The Air Force finally threw in the towel on August 21, 1957, canceling the entire XF-103 project.
Had the U.S. Air Force actually fielded an F-103, it would probably have been a supersonic waste. Fears that the Soviets had an armada of intercontinental bombers that could reach strike America proved to be unfounded (the infamous "bomber gap" myth). By the early 1960s, the Soviets were putting most of their nuclear eggs in the ICBM basket. There wouldn't have been many bombers to intercept, nor would the F-103 have been useful for the low-speed dogfights over Vietnam.
The XF-103 was an amazing design best left on the drawing board.