Wrong. It was designed by American engineers employed by North American Aviation, and approximately 15,000 A-36 & P-51's were built by American employees in Dallas and Los Angeles. The original design for the P-51 stemmed from a suggestion by the British Purchasing Commission in 1939 that North American Aviation establish a production line to augment production of the Curtis P-40. But the request wasn't presented with any urgency and wasn't taken seriously at the time.
However, "Lee" Atwood had been toying with the idea of designing a fighter at North American, and had examined the P-40 at various times and felt that a considerably better design could be developed.
In Jan 1940, the British Purchasing Commission renewed their suggestion that North American Aviation build P-40's. At which time Atwood went to the British Purchasing Commission and told them he could make a better plane. After some dickering went on, Sir Henry Self who headed the British Purchasing Commission, called Atwood front and center and told him and made not of the fact that North American never had designed a plane, but if Atwood could get ahold of wind-tunnel and flight test data, it would increase their confidence in North American Aviation's abilities in moving forward in a timely way.
Atwood then arranged to pay $56,000 to Burdett Wright, then general manager of the Curtiss Division for wind-tunnel and flight test data. This sum would reimburse Curtis their out-of-the-pocket expenses and a proportion of the costs of the tests. He then went back to Sir Henry Self, and presented him with a contract for 320 NA-73's, equiped with an Allison engine, certain armaments to be furnished by the British, and an airframe to be designed and built by North American Aviation, not to exceed $40,000 per plane.
At the time, no drawings or specifications were presented, except some free-hand sketches to demonstrate proof-of-concepts. The letter contract was the sole document in existance at the time. Neither did the original concept include the laminar airflow wing. This was incorporated into the subsequent design through work done by aerodynamics department of North American Aviation engineering group (the concept being origninated by Ed Horkey who was one of the aerodynamic specialists in the division).
By the spring of 1942 several test pilots had flown the P-51 and to a man were quite impressed by its handling and performance. For a fighter it was reported that it was remarkably docile, having light control forces, and the only complaint was that of weak aileron responses. This was all in context of a low/medium altitude fighter. However, above 25,000' the Allison struggled in the thin air.
The consensus was that the P-51 kicked anything in the sky's butt below 15,000 (including the Spitfire V - the standard RAF dayfighter). In April of 1942, Rolls Royce test pilot Ron Harker was invited to take the P-51 for a spin and when he did wheels down, he said "Its a bloodie good aeroplane, laddie; it only needs a little more poke."
And the idea to meld the Spitfire V's two-stage two-speed turbo-charged Merlin 61 with the P-51 airframe was born.
And as they say, the rest is history.