P-51 B/C (Mustang III)

Enter the Merlin

The X-factor

Design changes

Production expansion: is it a B or a C?

Later modifications

Operational service

Onto the legend


Serial numbers

3-Way drawings



Enter the Merlin

The lack of performance at high altitudes was a considerable problem in the Allison powered Mustangs but finding a quick solution was not very simple. Allison engines were pretty much all that was available to NAA at the time. A brief consideration was made to use the turbo-supercharged version of the Allison which was used on P-38s, but in the end NAA decided not to do so.

On April 30th, 1942, Rolls-Royce test pilot Ronald Harker visited the RAF Air Fighting Development Unit field at Duxford, and was invited to try out a Mustang. He flew the aircraft for a half-hour and was extremely impressed with it. The Mustang was 30mph faster than the Spitfire Mk V and had double the range. Eventually, Harker too ran into its poor high-altitude performance. He wrote a memo, recommending the pairing of the Merlin engine with the Mustang airframe. This caught the attention of Rolls Royce management and it wasn't long before four Mustangs were borrowed from the RAF for testing purposes.

Rolls-Royce looked at various Merlin variants such as the single stage Mk XX and the two-stage Mk 61. Engineers rejected a turbo-supercharger as too big and bulky, so they went for the two-stage supercharger scheme (the superchargers were stacked in series). The penalty of a two-stage supercharger was that temperature of the rapidly compressed air that passed through them increased by 200 degrees. To lower the temperature an intercooler was added, resulting in an extra radiator.

Studies for a Mustang with a Merlin engine suggested a top speed of 710 KPH (440 MPH) at 7.8 kilometers (26,000 ft) altitude, well in excess of the capabilities of an Allison Mustang, and possibly even superior than the Spitfire IX in some respects.

The plan was approved and on August 12th, 1942, four Mustangs (AM203, AM208, AL 963 and AL975) were delivered to the Rolls-Royce facility at Hucknall, were fitting of the Merlin 65 engine began. These Mustangs were redesignated as Mustang X.

The Merlin 65 was optimized for better performance at lower altitude, though unlike the Allison its performance did not drastically fall off at high altitude.

At the same time American engine manufacturer Packard had come to an agreement with Rolls-Royce in October 1942 to build the Rolls-Royce "Merlin XX" engine under license as the Packard V-1650-1, but supplies were limited, and there were valid doubts that the Merlin XX was that big of an improvement over the Allison.

In May 1942, Thomas Hitchcock, US military attaché in London , was informed by Rolls-Royce that they had plans to convert the Mustang airframes to the Merlin engine. Hitchcock had already flown the Mustang and had been thinking along a similar path, so he passed the word onto NAA.

Packard Merlin V-1650-3 engineMeanwhile, Packard had negotiated with Rolls-Royce to build a licensed version of the two-stage Merlins, and was moving into production of the Packard V-1650-3. The first V-1650-3 engines were to roll off Packard production lines in mid-December 1942, with production rising up to full volume in the following months. If NAA engineers wanted to use Merlin power, the engines would be available when they were needed.

NAA received green light to proceed with the project on July 25th, 1942. They received the two Mustang Is reserved for experimental purposes by the USAAF, mentioned in the previous chapter, and Merlin 65 engines sent from England . The P-51 serial numbers were 41-37350 and 41-37421.

NAA engineers threw themselves into the project with the same energy that they had shown in rolling out the initial NA-73X. The new variant was given the company designation of "NA-101" and military designation of "XP-78". By the summer of 1942, work on the Merlin Mustang was in full motion on both sides of the Atlantic.


The X-factor

The two groups were aware of each other's activities, and the spirit was cooperative. The Rolls-Royce group had a head start and got into the air first with their "Mustang X" with the initial aircraft flying on October 13th, 1942. The next day, Packard officials sent a letter to Rolls-Royce congratulating them on "beaten us to it on the flight of the Merlin Mustang. Hope performance is up to expectations."

The Rolls-Royce conversions were really experimental improvisations. The conversions featured a deep chin scoop underneath the prop spinner that was faintly reminiscent of late-model P-40s, and gave them a unique appearance among the Mustang family. The first Mustang X was originally fitted with a standard Rotol four-bladed propeller, as was then used by the Spitfire IX. This was later upgraded to a custom-built propeller, though it proved to have little influence on performance.

Mustang Merlin conversion
The British Merlin conversion (note the deep chin radiator scoop)

The performance was very satisfactory. The Merlin 65 could provide more horsepower at altitude than the Allison V-1710 did at takeoff, and the initial Mustang X conversion (AM208) achieved 697 KPH (433 MPH) at an altitude of 6.7 kilometers (22,000 ft). The aircraft could reach an altitude of 6.1 kilometers (20,000 ft) in 6.3 minutes, about two-thirds the time required by an Allison Mustang. AL975 tested an absolute ceiling of 40,600ft (12400m). It generated 200 more horsepower at 20,000 ft and 500 more horsepower at 30,000 feet than the Allison Mustang and it could outrun its predecessor by 100mph at 30,000 ft.

The second Mustang X flew on November 13th, 1942, and the third flew a month later. Rolls-Royce kept NAA informed of the results of the tests while NAA refined their own conversion, which had been redesignated "XP-51B" in the meantime.

The first XP-51B (41-37352) was flown by Bob Chilton on November 30th, 1942. As with the Mustang X, performance of the XP-51B demonstrated that faith in the Merlin conversion was justified. NAA test pilot Bob Chilton achieved a level speed of 710 KPH (441 MPH) at an altitude of 9 kilometers (29,800 ft), and the XP-51B could climb almost twice as fast as an Allison Mustang.

XP-51B 41-37352
XP-51B 41-37352 was originally built as a P-51 for the RAF but was taken over by the USAAF. The 20mm cannon armament was retained. Note the changed cowling as a result of the Merlin engine installed (photo by USAF).

This first XP-51B was roughly an "80% conversion", but the second XP-51B (41-37469) , which flew soon afterward, was closer to a production design. The Merlin 65 had a similar physical envelope to the Allison V-1710, but weighed about 136 kilograms (300 pounds) more, and required fitting the intercooler someplace in the fuselage.

Left: XP-51B windtunnel tests (photo by NASA) - Right: XP-51B (photo by USAF)

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