Nope, it was just available to those who needed it when the war started. The Mustang was designed after the war started and started as the A-36 (another pig) and was made the war winning fighter after being married to the Rolls Royce Merlin engine.
From what I’ve seen on the history channel, the Mustang was designed just before the outbreak of the war in conjunction with the British. It became a great plane when they put the Rolls Royce Merlin engine in it with a supercharger in it.