The Development of the P-51B/C

Not to be daunted by the shortcomings of the P-51A, North American set out to fulfill the Mustang’s mission of being the most capable fighter in the skies over Europe. At the recommendation of the air attaché at the U.S. embassy in London, North American designers began by pulling the Allison powerplant and substituting the Rolls-Royce Merlin engine. The Merlin had for some time been produced by Packard in the United States, and North American relied primarily on Packard Merlins to power the new Mustang, which would receive the designation of P-51B/C. Aircraft manufatured at North American's Inglewood, CA plant were designated P-51B's, while those built at NAA's new Dallas plant were P-51C's. The aircraft built at Dallas recieved minor improvements over the "B" model, warranting its separate designation. Compared to the Allison V-1710, producing 1200 horsepower, the Rolls-Royce/Packard "Merlin" superturbocharged V-12, the V-1650, would provide 1,720 HP. This additional power would drive the Mustang’s top speed up to 437 MPH. It would also give the Mustang an initial rate-of-climb of 2,600 feet-per-minute. Together, these performance improvements would make the P-51B/C an extremely capable defensive fighter. It would be faster than both the Me-109 and the FW-190; it could out-climb both; it could out-dive both; it could out-turn both.

Although the R.A.F. would have welcomed this version of the Mustang, the newfound interest and subsequent involvement of the Army Air Force in the Mustang, led to the Mustang receiving further improvements first. The American Eighth Air Force had begun bombing targets in Festung Europa on August 17, 1942. From that time on, the AAF would want fighters with increasingly long range to escort bombers over hostile target areas. The AAF needed a means of countering the packs of Luftwaffe Me-109s and FW-190s which would pounce on bomber formations. They needed an aircraft capable of defending the bomber formations that had the range to accompany them all the way to the target area. For a time, the idea of building an entirely new fighter was discussed. Due to past problems with external drop-tanks catching on fire when jettisoned, designers were reluctant to employ them. In order to carry sufficient fuel without drop-tanks, AAF planners theorized, the new fighter design would have to be so large that it would not be much more maneuverable than the bombers it was escorting and it would be as expensive as the bombers themselves. This obviously was an unacceptable remedy.

Under directive from the commander of the Army Air Force, General Harold "Hap" Arnold, the AAF would begin to attempt to extend the range of its fighters in an effort to give the bombers better protection. In July of 1943, P-47 Thunderbolts were outfitted with drop-tanks and this extended their range to 260 miles from base. In the initial series of missions, this surprised the Luftwaffe and accounted for a fairly respectable number of kills. At first, Goering refused to believe that Allied fighters were able to reach Germany. Despite the fact that a P-47 had been shot down over the Reich, he actually ordered his adjutants that it was impossible. He would have to accept the facts, however, as P-47 escort out to a 260 mile radius became a reality. The Luftwaffe, in response, simply held off their attacks on the bomber formations until the Thunderbolts had to turn back. It was apparent that the 260 miles now capable by the P-47 was in no way satisfactory.

Noting that the P-51A had been the first fighter to fly over the Reich, Hap Arnold gave priority to the P-51 project under the stipulation that its range be further extended. The motivation behind the redesign of the Mustang became not defense-oriented, as the R.A.F. wanted, but instead the new Mustang would be tasked as an escort fighter responsible for acquiring air superiority over Germany. As a means of increasing both fuel economy and performance, North American did in fact install the Rolls Merlin. The Merlin burned fuel at approximately half the rate of the P-47’s big radial engine. The problem that North American soon discovered with installing the Merlin was that the copper tubing for the Merlin’s cooling system was not compatible with the Mustang’s aluminum radiator. The problem was quickly solved with advice from the Bureau of Standards in Washington. Using the same method which isolates beer from its metal can, North American coated the radiator with a lacquer, which prevented corrosion and enabled the marriage of Mustang to Merlin.

To augment its already long range, North American added an eighty-five gallon fuel tank just behind the pilot’s seat. This placed an additional 650 pounds aft of the center of gravity, but instead of reconfiguring the aircraft, pilots were told to burn fuel out of the fuselage tank first and to simply fly more carefully until the aircraft’s moment had shifted over the center of gravity. The designers then proceeded to install hard points on the wings for drop-tanks. With its 269 gallons of internal fuel, the Mustang’s range was then extended first with a pair of seventy-five gallon drop-tanks and subsequently with two 108 gallon external tanks.

These improvements were settled upon and the aircraft, the P-51B/C Mustang, began arriving September 17, 1943. With its more powerful Merlin, all the performance deficiencies were remedied, in fact some were "over-remedied". The 1,720 HP Merlin, coupled with the Mustang’s Hamilton-Standard four-blade propeller and a high-speed gearbox, provided so much torque to the relatively light Mustang that if full throttle was applied on take-off without simultaneously applying full left rudder, the torque would literally flip the aircraft over on its back! This impressive display of power, coupled with the superturbocharger’s ability to provided higher speed at altitude and higher rate-of-climb, righted the wrongs North American had done by underpowering the "A" model.

This newfound performance was coupled with amazing range. On internal fuel alone, the Mustang was capable of about 220 miles, almost what the Thunderbolts were capable of with drop-tanks. With the seventy-five gallon external tanks, the Mustang’s range shot up to 650 miles. With the pair of 108 gallon tanks, the Mustang was capable of flying 850 miles. With this range, the Mustang was able to accompany bombers anywhere in Greater Germany. Hap Arnold had realized his quest for a plane to save his bombers.

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