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This prototype supersonic United States Navy fighter was built in 1977. The XFV-12 design attempted to combine the Mach 2 speed and AIM-7 Sparrow armament of the McDonnell Douglas F-4 Phantom II in a VTOL (vertical takeoff and landing) fighter for the small Sea Control Ship which was under study at the time. On paper, it looked superior to the subsonic Hawker Siddeley Harrier attack fighter. However, it was unable to demonstrate an untethered vertical takeoff and its inability to meet performance requirements terminated the program.

In 1972, the Navy issued a request for proposals for a next generation supersonic V/STOL fighter/attack aircraft. Rockwell's design with the XFV-12 won against Convair's proposal with the Convair Model 200. The XFV-12A, despite its concept being considered risky compared to that of the Harrier, was selected for development.

To reduce costs, the nose from a Douglas A-4 Skyhawk and intakes from the F-4 Phantom were used. Engine rig testing began in 1974. Free-flight model tests conducted at the NASA Langley full-scale wind tunnel showed the projected thrust augmentation levels were highly optimistic, and that the aircraft would most likely be incapable of vertical flight on the thrust available, while the design remained suitable for conventional flight.

The XFV-12 used a thrust augmented wing concept in which exhaust would be directed through spaces in a wing opened up like venetian blinds to increase available lift, somewhat like Lockheed's unsuccessful XV-4 Hummingbird. Such arrangement restricted weapons carriage to under the narrow fuselage and two conformal missile mounts. Its canards were extremely large, with almost 50% of the area of the wings, making it effectively a tandem wing. The 30,000 lbf (130 kN)-class afterburning turbofan engine had enough thrust to lift the weight of the 20,000 lb (9,072 kg) aircraft. It was modified to further increase thrust for vertical lift. The rear engine exhaust was closed and the gases redirected through ducts to ejector nozzles in the wings and canards for vertical lift.

Ground testing of the XFV-12A began in July 1977, and the aircraft was officially rolled out at the Rockwell International facility in Columbus, Ohio on 26 August. Due to increasing costs, the construction of the second prototype was abandoned.

Tethered hover tests were conducted in 1978. Over the course of six months, it was determined that the XFV-12A design suffered from major deficiencies with regard to vertical flight, especially a lack of sufficient vertical thrust. Lab tests showed 55% thrust augmentation should be expected; however, differences in the scaled-up system dropped augmentation levels to 19% for the wing and a mere 6% in the canard. While the augmenters did work as expected, the extensive ducting of the propulsion system degraded thrust, and in the end the power-to-weight ratio was such that the engine was capable of vertically lifting only 75% of the weight of the aircraft in which it was mounted.

Following the tests, and with the program suffering from cost overruns, the Navy decided the XFV-12A was not worth further development and canceled the project in 1981.

Specifications
Crew: 1
Length: 43ft 11in (13.39 m)
Wingspan: 28ft 6.25in (8.6932 m)
Height: 10ft 4in (3.15 m)
Wing area: 293 sq ft (27.2 m2)
Empty weight: 13,800lb (6,260 kg)
Gross weight: 19,500lb (8,845 kg)
Max takeoff weight: 24,250lb (11,000 kg)
Fuel capacity: 2,763L (730 US gal; 608 imp gal) in two fuselage bladder tanks and two integral wing tanks
Powerplant: 1 × Pratt & Whitney F401-PW-400 afterburning turbofan engine, 30,000lbs with afterburner.
Maximum speed: Mach 2.2-2.4
Thrust/weight: 1.5 (conventional)
Take-off run: 300 ft (91 m) at 24,250lb (11,000 kg)
Guns: 1 20mm M-61 Vulcan cannon w/639 rounds
Missiles: 2 AIM-7 Sparrow (carried under fuselage) and 2 AIM-9L Sidewinder AAMs or 4 AIM-7s