The P-51H was the end result of a program at North American to cut weight off' the Mustang so as to make it more competitive with the Spitfire and Messerschmitt in rate of climb. A lightweight fighter study, combined with combat reports indicating problems that P-51 pilots were encountering and test results from three experimental Mustangs - XP-51F, XP-51G and XP-51J - eventually evolved into the production lightweight P-51H.

The test weight of the XP-51F was to be kept below 5700 Ibs, down 1300 lbs from the P-51D. It was accomplished through the use of a much thinner cross-section wing, a redesigned bubble type canopy, a much smaller landing gears rut and wheel, and an Aeroproducts prop of lighter weight and three blades.

The engine was the proven Packard V-1650-3 as used in the P-518. The XP-51F weighed in at 5635 lbs and test flights resulted in a top speed of 466mph at 29,000ft and a rate of climb of 4,000ft per minute.

The XP-51G was an XP-51F airframe fitted with an experimental Rolls Royce RM- 14SM engine that featured a very touchy fuel control metering device in place of a carburetor. When the device was 'right' it would develop over 2,000hp at 20,000ft. Top speed for the XP-51G rose to 495mph and a climb rate of over 5,000ft per minute was attained. Due to the fact that the cockpit was not pressurized, the service ceiling was limited to 46,000ft. The XP-51J was a stretched lightweight airframe built around the new Allison V-1710-119, which was supposed to be equal to the special Merlin RM-14SM. Tests were flawed by many problems with the new engine. Eventually the problems were ironed out and the engine went on to power all P-82 models beyond the E.