[fred cook]

R.J. Mitchell was the lead designer at Supermarine and worked on the Spitfire's development until his death in 1937. He had traveled extensively in Germany in the mid 1930's and saw what the Germans were doing to develop an overwhelming air force. He knew that England would need a state of the art fighter plane if it was to survive a war with Germany. After Mitchell's death, his colleague (Joseph Smith) took over the development of the Spitfire. The Spit was adopted into the Royal Air Force in 1938. There were a number of different models of the Spitfire developed during the war, each being better in some way than those that came before.

The Mustang was originally designed to use an Allison V12 engine but it's performance was lacking especially at high altitudes. Since the Allison engine was very much like the Rolls Royce Merlin engine, putting the supercharged Merlin into the Mustang frame was fairly easy. The addition of the compressor cured the problems being experienced at high altitudes. I suspect that it was American assembly lines that made the P51 less expensive to build than the Spit. Both the P51 and the Spitfire had very good kill ratios.