The designation of RAF squadrons early in World War II in which American pilots served. The three Eagle Squadrons were 71, 121, and 133 under Royal Air Force Fighter Command. The Eagle Squadrons were commanded by British pilots (16 of them), but contained 244 American pilots.

These 260 pilots had 73 1/2 aerial victory credits, at a loss of 77 American and 5 British pilots. The Eagle Squadrons were active from September 1940 until September 1942, at which point they were disbanded and incorporated into the USAAF as the 334th, 335th, and 336th Fighter Squadrons of the Fourth Fighter Group.

Information extracted from http://www.fourthfightergroup.com/eagles/es.html