The P 51 was the most versatile fighter plane of WW2. It excelled at both ground level support of the troops and high altitude long range fighter cover for the heavy bombers. It was the equal of the best fighter planes in dog fights. It was also an accident of fate or timing rather than planning that it existed.
In late 1940 or early '41 Britain was desperately short of fighter planes of any sort and had sent some RAF people to the US to see what they could buy. Hoping to buy some P-40's, they talked with Curtis Wright, but found all the P-40's booked up by the US govt. They got a tip from Curtis Wright that North American Aviation had no contracts yet and was looking for business, and that Curtis Wright would license North American to build P-40's if they worked out an agreement. When the RAF asked the small North American management crew if they would be interested, their answer was, "why do you want to buy an expensive high priced plane that would take well over a year to start any production when we have a new design worked out and can have a prototype ready for RAF test flights in 9 months, if you can get us an engine for it from Allison." They made a deal and were very happy with the performance of the prototype and placed a production order. When this news worked its way through to the Army Air Force, they decided to look into the possibilities and eventually ordered a production run that wound up as the A-36 low level fighter. The Allison V12 was the only non-radial combat aircraft engine built in the USA, and unlike the Rolls Royce Merlin engine and the German ME 109, it did not have a supercharger, essential for combat above 15,000 ft altitude as well as top performance at lower altitudes. After the RAF had a few months experience with the new plane, the RAF did the obvious thing and grafted a Rolls Royce Merlin engine from a Spitfire onto one of their P-51's. At this time the RAF and the North American folks discovered that Packard Motor Company in the US was building Rolls Royce engines for the PT Boats of the Navy. Packard was happy to add Merlin engines for North American to its production runs and the
P-51 was officially launched.
One of the benefits of North American being small and nearly broke was that the design engineers had to do double duty as production engineers, so they designed the plane for simple and fast (as well as low cost) production. At that time the conventional practice was to build the airframe and then have people crawl inside to string all the wires and cables and this more than doubled the assembly time and cost. North American designed the fuselage in two half shells and had the electrical, hydraulic and control cables, etc., installed in each half prior to sewing the halves together. The P-51 cost about one third of the P-38 and about half the cost of the P-47 and F6F Navy planes.
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