Hey FF, not just one unit of the USAAF flew Spits. It started with American nationals in the RAF in 1940 -41, in three Squadrons, which then became American Sqns in the RAF, the 'Eagle' Sqns. These in turn became the 4th Fighter Group, 8th AF.
There were a number of units in US forces flying Spits, throughout the war, including Recce and US Navy spotters, the latter during 'Overloard'.
If you wish to learn more, I'll gladly point you in the right direction.
Terry.