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The P-51 Mustang is one of the most famous fighter aircraft of the Second World War. It was developed at great speed to British specifications in the dark days of 1940, and earned its great fame during the allied bombing campaign over Germany in 1944. Although not the first American fighter to escort bombers to Berlin (that honour goes to the P-38 Lightning), the Mustang often gets the credit for that feat, possibly because Hermann Goring had stated that once he saw the Mustangs over Berlin, he knew the war was lost.
The Mustang story began in January 1940. Britain and France urgently needed modern fighter aircraft. Both countries had suitable aircraft of their own, but not in sufficient numbers. Both turned to American industry for a quick solution to the problem. Britain wanted to increase their order for the Curtiss P-40, but Curtiss had no capacity for extra production. The British Purchasing Commission went to North American Aviation, hoping to negotiate a license deal for production of the P-40.
North American did not want that contract. Instead, they were convinced that they could produce a better aircraft than the P-40, based on the same engine. This was an appealing prospect. The P-40 was a capable aircraft, but it was not in the same league as the Supermarine Spitfire or Messerschmitt Bf 109. However, in early 1940 North American Aviation had no great reputation – they were best known for the T-6 Texan trainer (Known as the Harvard in RAF service), while the B-25 Mitchell bomber had yet to enter active service. Despite this, on 10 April 1940 the BPC accepted the North American proposal. Two weeks of intense work followed, and on 24 April the first design drawings were accepted by the BPC.
The new aircraft had to be built to British requirements. That meant using the Allison V-1710 inline engine, as used in the P-40 and later the P-38 Lightning, although without the turbo-supercharger used in the P-38. Each aircraft had to cost under $40,000. The new aircraft would have all of the features seen as most desirable in early 1940 amongst them a stressed skin construction and self sealing fuel tanks. However, it would also feature two new high performance technologies.
Part of the credit for these advanced features must go to Lieutenant Benjamin S. Kelsey, head of the Army Air Corps Pursuit Projects Office. On 4 May 1940 North American had been given permission to sell their new NA-73 fighter to the British, as long as they provided two aircraft to the USAAC. In return Kelsey took an interest in the development of the new aircraft.
Kelsey was in a position to know what research would be most useful to North American in their work. He was directly responsible for posting Eastman Jacobs to the project. Jacobs was an expert on laminar flow airfoils, and was then working for the National Advisory Committee on Aeronautics. Laminar flow wings had a different cross-section to standard aircraft wings, with similar curves on the upper and lower surfaces, and the thickest part of the wing set much further back than was normal. In theory this produced a smoother air flow over the wing, reducing drag at high speed, although in practice problems with construction quality tended to reduce the impact of laminar wings – every protruding rivet or unsealed crack would damage the laminar flow.
Second, with official backing North American were able to purchase all of Curtiss’s design work for the P-40 and XP-46. This later led to accusations that North American stole much of the design of the Mustang, accusations given more credibility because North American had to use Curtiss’s wind tunnel. If this had been the case, then one would have expected better performance from the P-40.
One idea that was taken from the original design for the P-40, and from the design for the XP-46 was the location of the radiator scoop. On the P-40 this was placed just under the nose, giving the aircraft its distinctive shark’s mouth, but also causing significant levels of drag. It had originally been planned to place this radiator scoop under the wings, where it could be made wide and flat, reducing drag.
North American took this design one step further. Their air scoop had a vent at the back used to expel hot air. This produced a limited ram-jet effect, which made up for the drag imposed by the air scope and even produced a limited amount of extra thrust at some speeds!
With help from the NACA and Curtiss, and intense hard work, North American had the airframe of the prototype Mustang ready by 9 September 1940. The only problem was that the engine had not yet arrived! That too was in place by the end of September. On 11 October 1940, the new aircraft made its first taxi tests, and on 26 October took to the air for the first time.
One side issue with the development of the Mustang is the number of days it took to produce the airframe. 107 days is often quoted. However, this would only take us back to 25 May. From 10 April, when the British had officially ordered the Mustang, to 9 September when the first aircraft was wheeled out was actually a period of 153 days. However, assuming a five day working week, there were 107 working days between 24 April and 9 September.
The NA-73X had a short life. On 20 November it suffered serious damage after a crashing landing. It returned to the air on 11 January 1941, but was finally retired on 15 July, after having made 45 flights. Early flight tests proved that the new design was a success. The beautifully streamlined airframe, laminar flow wings and advanced radiator resulted in an aircraft that outperformed the P-40, despite using the same engine. The Mustang I would have a top speed of 382 mph at 13,000 feet, well up on the 352 mph at 15,000 ft of the P-40B.
The new design only had two faults. The first was that the Allison engine with its single speed supercharger lost power at high altitude. This was already known when the aircraft was designed – it had after all been ordered as an improvement on the P-40, not a replacement for the Spitfire. Its low level performance was excellent – the Bf 109E could only managed 348mph at 14,560ft.
The second flaw was more serious, and would not be solved until the appearance of the P-51D. The view from the cockpit was seriously restricted, with a huge blind spot to the rear. During the Second World War good visibility was one of the most important features of any fighter aircraft, with most “dogfights” over before the loser knew he had been spotted.
Despite these limits, the P-51 Mustang was clearly an excellent aircraft. The British order was soon joined by an American order for 150 aircraft. The Allison-powered P-51, P-51A, Mustang I and Mustang IIs performed invaluable service for both the RAF and the USAAF, specialising in ground attack and photographic reconnaissance work. Work on producing a high altitude Mustang, powered by the Rolls Royce Merlin engine began in the middle of 1941. The Mustang was well on its way to earning its reputation as one of the great aircraft of the Second World War.