The P-40Q was an experimental project which attempted to produce a really modern fighter out of the existing P-40. The modifications were in fact so drastic that there was very little in common with earlier P-40 versions.

Two P-40Ks (serial numbers 42-9987 and 42-45722) and one P-40N (serial number 43-24571) were extensively modified with revised cooling systems, two-stage superchargers, and structural changes which markedly altered their appearance. The project was assigned the designation XP-40Q.

The first XP-40Q was P-40K-10-CU ser no 42-9987 fitted with a new cooling system, a longer nose, and a four-bladed propeller. The radiators were moved into an under-fuselage position, with intakes between the undercarriage legs.

The most prominent XP-40Q feature, used on 42-45722 and 43-24571, was the cutting down of the rear fuselage and the addition of a bubble canopy as on the "XP-40N". Later the wingtips were clipped. The result was an aircraft which bore almost no resemblance whatsoever to its parent P-40 line. The V-1710-121 engine was fitted with water injection, resulting in a power of 1425 hp. Speed increased to 422 mph at 20,500 feet, making it the fastest of all the P-40s. An altitude of 20,000 feet could be reached in 4.8 minutes, and service ceiling was 39,000 feet. Four 0.5-inch machine guns were carried by the prototypes. Wingspan was 35 feet 3 inches (after clipping), and length was 35 feet 4 inches (2 feet longer than the P-40N).

The proposed production models of the P-40Q were to have carried either six 0.50-inch machine guns or four 20-mm cannon, but the XP-40Q was still inferior to contemporary production Mustangs and Thunderbolts, and development was therefore abandoned. Consequently, the production life of the P-40 ended with the N version.

The second XP-40Q was briefly used for postwar air racing. Registered NX300B, the second XP-40Q was an unauthorized starter in the 1947 Thompson Trophy race. It was in fourth place when it caught fire and had to drop out of the race.