Americans in the RCAF
The US did not enter the war until December 1941, following the Japanese attack on Pearl Harbour. Many Americans did not wait for this declaration of war and instead opted to join the Allied Forces.
One story is that of P/O Claud Weaver III, DFC, DFM, an American, born at Oklahoma City, Okla., on August 18, 1922. He came to Canada to enlist in the RCAF at Windsor, ONT., on February 13, 1941. He earned his wings in October 1941, went overseas at once, and after a brief period with a fighter squadron in Britain was posted to Malta. There he flew with No. 185 Squadron from July to September, 1942.
In August, Sgt Weaver was decorated with the DFM, for destroying five enemy fighters and participating in a bomber kill within a period of one week. He ran his Malta score up to ten before being shot down over Sicily and taken prisoner on September 9, 1942. A year later he escaped from the Prisoner of War (POW) camp and walked 300 miles to freedom. Appointed to a commission, he immediately returned to operations with No. 403 (RCAF) Squadron in Western Europe, late October 1943. He won two more victories before he was shot down and killed in air combat while on a “ranger” mission in the Amiens area on January 28, 1944. March 1944, the award of the DFC was published and P/O Weaver also was mentioned in dispatches in June 1944.
Source
Edited: 15000 Americans joined RAF and RCAF prior to starting of WW2 for USA (according to http://www.wpafb.af.mil/museum/history/prewwii/es.htm )
Second edit:
Many American pilots served in the Royal Air Force and in order to circumvent the US Neutrality Act they assumed Canadian or South African nationality. They formed the Eagle Squadrons, approved by the British Air Ministry in September, 1940, and operated within the RAF Fighter Command. The first Eagle Squadron was No. 71 Squadron, formed with Hurricanes at RAF Station, Kirton-in-Lindsay, Lincolnshire. The ultimate total of US pilots thus serving numbered 243 with additional squadrons Nos.121 and 133 operating from Kirton-in-Lindsay and Coltishall respectively. After the US entry into the war the Eagle Squadrons were transferred into the US 8th Air Force.
My bold. Source