The Cougar was the U.S. Navy's first swept wing, carrier-based, fighter jet. The XF9F-2/XF9F-3 Panther contract awarded in October of 1946 had included a clause calling for design data on a swept-wing version of that fighter. Grumman, worried about the poor low-speed characteristics of swept-wing aircraft, prevailed upon the U.S. Navy to postpone procurement of a swept-winged version of the Panther. Development of a swept-wing Panther became more urgent as MiG-15s appeared in the skies over Korea in November of 1950. The swept-wing version of the Panther was designated F9F-6, but it was given a different name-Cougar. This continued the tradition of assigning feline names to Grumman-built fighter aircraft. It remains something of a mystery why the navy did not renumber the Cougar as the F11F-1, which was the next numerical designation available.

The first F9F-6 (BuNo 126670) was ready for its first flight only six months after the contract was signed. The Cougar flew for the first time on September 20, 1951. The National Air & Space Museum's F9F-6 (BuNo 126670) was the first prototype built by Grumman.

The Cougar was the U.S. Navy's first swept wing, carrier-based, fighter jet. The XF9F-2/XF9F-3 Panther contract awarded in October of 1946 had included a clause calling for design data on a swept-wing version of that fighter. Grumman, worried about the poor low-speed characteristics of swept-wing aircraft, prevailed upon the U.S. Navy to postpone procurement of a swept-winged version of the Panther. Development of a swept-wing Panther became more urgent as MiG-15s appeared in the skies over Korea in November of 1950.

The MiG-15 was powered by derivatives of the same Rolls-Royce engine as was the Panther, but was nearly 100 mph faster. By Christmas 1950, the Navy and Grumman both agreed that it was urgent to accelerate the development of a swept-wing version of the Panther. A contract for the modification of three F9F-5 airframes was signed on March 2, 1951. After some numerical evolution, the project was designated "Design 93."

Grumman's Design 93 was, essentially, a swept-wing conversion of the F9F Panther. It retained the fuselage, vertical tail, engine, and undercarriage of the F9F-5, but was fitted with wings and horizontal tail surfaces swept at 35 degrees. In order to improve slow-speed flight characteristics, the chord line of the wing when configured with extended leading-edge slats and trailing-edge flaps was increased. To accomplish this, larger split flaps were fitted underneath the fuselage center section. The fuselage was lengthened by two feet, the wing root-mounted intakes were extended farther forward, and the wing root fillets were enlarged. The upper rudder section was unchanged, but was linked to a yaw damper. Due to adverse impact upon performance, wingtip tanks had to be eliminated. The resulting reduction in fuel capacity was partially offset by increasing the size of the forward fuselage fuel tank and by adding bladder-type fuel tanks in the wing leading edge. The internal fuel capacity of the swept wing design shrunk by nearly 1000 lb. from 1003 US gallons for the F9F-5, to 919 US gallons.

The swept-wing version of the Panther was designated F9F-6, but it was given a different name-Cougar. This continued the tradition of assigning feline names to Grumman-built fighter aircraft. It remains something of a mystery why the navy did not renumber the Cougar as the F11F-1, which was the next numerical designation available.

Two flying prototypes (126670 and 126672) and a static test airframe (126671) were obtained by converting three F9F-5 airframes to the F9F-6 configuration. Work on the swept-wing Cougar proceeded quite rapidly. The first F9F-6 (BuNo 126670) was ready for its first flight only six months after the contract was signed. The Cougar flew for the first time on September 20, 1951.

A Pratt & Whitney J48-P-6 turbojet engine rated at 6250 lb. thrust (dry) and 7000-lb. thrust (water injection) powered the F9F-6. It had conventional horn-balanced ailerons for lateral control and conventional tab geared elevators for longitudinal control. Early test flights revealed that the F9F-6 had a tendency towards control reversal at high speeds, and had rather poor lateral and longitudinal control-common problems for early swept wing aircraft. The adoption of an all-flying horizontal tailplane cured the reversibility problem while the addition of "flaperon/flaperette" spoilers fitted to the upper wing surfaces solved the lateral control problems. In normal flight, both of these spoiler sections operated as a single unit, but the flaperette section could operate independently. Large wing fences were found necessary to inhibit span-wise airflow and to preserve lateral control effectiveness.

After these modifications were made, the prototype F9F-6 actually had better carrier handling characteristics than the straight-winged F9F-5. The critical Mach number was increased from 0.79 to 0.86 at sea level and to 0.895 at 35,000 feet. The prototype (126670) was later re-engined with a YJ48-P-8 turbojet rated at 7250 lb. thrust and no longer needed water injection. After the first 30 aircraft were built with the J-48-P-6A, the rest were fitted with the 7250-lb. thrust J48-P-8 turbojet. Armament and stores consisted of four 20-mm M3 cannons and two wing racks that could be loaded with up to 3000 lb. of bombs or 150-US gallon drop tanks. The first unit to receive the F9F-6 was VF-32, which converted to the Cougar in November of 1952, too late to fly combat sorties in Korea. The last of 646 F9F-6 Cougars was delivered on July 2, 1954.

In service, F9F-6s were often fitted with a UHF homing antenna in a fairing underneath the nose. Several had an in-flight refueling probes installed in the nose. Three probe-equipped F9F-6s flown by VF-21 made the first US transcontinental crossing in less than four hours on April 1, 1954, the fastest time being 3 hours 45 minutes 30 seconds for a distance of 2438 miles. Sixty F9F-6 Cougar airframes were fitted by Grumman with a camera installation in the nose in place of the cannon, and were delivered under the designation F9F-6P between June 1954 and March 1955. The nose was slightly longer (the length increasing from 41 feet 5 inches to 42 feet 1 7/8 inches). This reconnaissance version was unarmed.

Following the withdrawal of F9F-6s from active service, several F9F-6s were modified drones. The F9F-6K2 designation identified F9F-6Ks that were equipped with modernized equipment, as a well as a number of F9F-6s, which were fitted directly with this newer equipment. The designation F9F-6PD was assigned to two F9F-6Ps (BuNos 127475 and 128308) that were modified as target controller planes.

In 1962, the Defense Department introduced the new Tri-Service aircraft designation scheme under which the separate USAF/Navy designations were replaced by a new joint system. This required the redesignation of all navy aircraft. The F9F-6 was redesignated F-9F.

The National Air & Space Museum's F9F-6 (BuNo 126670) was the first prototype built by Grumman.