P-51 H (Mustang V)

Getting there: XP-51F, G & J

Final result: the P-51H

Design changes


Too late for combat


Serial numbers



Getting there: XP-51F, G & J


In July of 1943, NAA suggested to the USAAF that they design and build a special lightweight version of the P-51. This was done to make the Mustang more competitive with the Spitfire and Messerschmitt in rate of climb. For this reason, Edgar Schmued, chief of design at NAA, left the US on a 3 month trip to Britain . There he worked together with Supermarine and Rolls-Royce to try and work out why the Spitfires were so much lighter than the Mustangs.

Rolls Royce had been working on a new Merlin, the RM.14.SM which increased horsepower to 2,200 and Schmued was very anxious to use this new engine. The people at Supermarine from their hand had no detailed weight data on the Spitfire, so they started weighing all the parts they could and made a report for Schmued.

From the start, the Mustang was designed to take on higher loads than the Spitfire. Also, British design standards were not as strict as the US design. As a result, the Mustang was much heavier than the Spitfire.

Upon his return to the US , Schmued began working on the project, which was given the designation NA-105. Weight was shaved wherever possible, bringing down the empty weight by 600 pounds. The emphasis was on weight reduction, simplification of all systems, improved maintenance and performance enhancement.

Since the structure of the aircraft was almost completely redesigned, the new lightweight Mustang had few parts in common with earlier versions. Most significant changes were:

  • A more streamlined wings with smaller wing tanks (386 liters of 102 US gallons). The wing had a new aerofoil with an even newer low-drag laminar flow profile. It was slightly larger and had a straight line leading edge, removing the kink that distinguished the D-model.
  • A longer but more aerodynamically efficient bubble canopy .
  • The cockpit layout was improved (standard British panel was adopted) and the pilot's back armor was integrated into the seat.
  • A redesigned landing gear with smaller wheels and tires. New brake discs were installed.
  • The fuselage tank was deleted.
  • The inboard wing guns were deleted, reducing armament to two 0.50 caliber guns in each wings (440 rounds each).
  • A three-bladed Aeroproducts hollow-steel propeller.
  • Simplification of the engine mounting and hydraulic system.
  • The engine coolant and intercooler radiators were redesigned and installed in a completely new duct which had a vertical inlet. It was also placed further away from the fuselage.
  • The oil cooler was removed from the radiator group, removing the long and vulnerable oil pipes. The oil passed through a heat exchanger mounted on the front of the oil tank and next to the engine intercooler. The flow of glycol carried away the heat from the oil.
  • Control surfaces were improved and the tail fin was enlarged.
  • Minor metal parts were replaced by molded plastic parts.

Two prototypes were ordered as XP-51F (43-43332/42-43334), with the contract amended in June of 1943 to five XP-51Fs. NAA designation was NA-105. The British requested that two of these aircraft be given to them for evaluation. Engineering inspections were held in February of 1944 and the first XP-51F was piloted by Bob Chilton and took to the skies on February 14th, 1944.

XF-51F prototype (Photo by NAA)
XP-51F prototype

It reached a top speed of 466 mph (750 KPH) at 29,000 ft. This was not really as much of an improvement the people of NAA were looking for, given that the empty weight of the XP-51F was 2,000 pounds less than that of the P-51D. Normal range was 650 miles with a maximum range of 2,100 miles. Directional stability was still a big issue however.

The third XP-51F was shipped to the UK on June 20th, 1944. It was painted in RAF camouflage and named Mustang V (serial FR409).



Before any construction begon on the XP-51F, it was agreed that the last two NA-105 airframes would be configured with the Rolls Royce Merlin 145M. These two Mustangs were designated XP-51G (43-43335/ 43336). Apart from the new engine, the only other change that was made was the installation of a five-bladed wooden Rotol prop.

XP-51G prototype (Photo by NAA)
The XP-51G prototype with Merlin 145M engine and 5-bladed wooden Rotol propeller

Work began in January of 1944 and the first XP-51G flew on August 9th , 1944, piloted by Ed Virgin and again by Bob Chilton on August 12th . The second took to the skies on November 14th later that year and was afterwards sent to the UK (renamed Mustang V (FR410)).

The five bladed propeller soon proved to be unsatisfactory and was replaced by the Aeroproducts four-bladed propeller.

It soon became apparent that this was the hottest Mustang yet, the second aircraft reaching a top speed of 495 mph (796 kph) during tests at Boscombe Down. Service ceiling was 45,700 ft.

One of the XP-51Gs achieved a top speed of 796 KPH (495 MPH) some months later, making it the fastest of all the Mustangs. An XP-51G was provided to the RAF for flight tests. It was not produced due to limited availability of the Merlin 145M.

Neither the XP-51F not XP-51G went beyond the prototype stage. The both however played a major part in the development of the end result, the P-51H.



The last prototype in the lightweight NA-105 series was the XP-51J. It was very similar to the F & G-models except it had the reintroduced Allison V-1710 engine. This Allison had a two-stage supercharger to improve high altitude performance and a water-injection system for boost power.

The Allison had an updraft carburetor which resulted in substantial modification of the upper nose of the Mustang. All air inlets in the nose were completely eliminated, resulting in a clean and streamlined nose.

The carburetor air was taken in through a ram air inlet at the front of the radiator duct and piped to the engine. A dorsal fin was also fitted.

XP-51J prototype (Photo by NAA)
The XP-51J prototype. Note the dissapearance of all air inlets in the nose and the dorsal fin.

Two XP-51J prototypes (44-76027/76028) were built, but only one of them actually flew with Joe Barton at the controls on April 23rd , 1945. A maximum speed of 491 mph was anticipated, but was never reached because the Allison had not been cleared for full power operations.

The end of the war in the Pacific brought further development of the XP-51J to an end.

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