Before the United States' entry into WWII, many American pilots volunteered for service in the RAF and RCAF.
As activity in the Battle of Britain began to gain a lot of attention in October of 1940, Americans came to the British call to fly Hurricanes and Spitfires and fill the depleted ranks in the RAF. Nearly 250 men were selected by the Knight Committee to fly in what became known as the "Eagle Squadrons."
Part of the Royal Air Force Fighter Command, the squadrons were numbered 71, 121 and 133. Their squadron and flight commanders were British RAF men. The training for these Yank aces was brisk. Once they arrived in England, the new recruits were sent to an operational training unit (OTU) for two to four weeks, where they learned to fly Miles Master trainers, Hurricanes, and Spitfires before being posted to a squadron. Once they arrived at their new squadron, they would learn to fly in formation as well as the squadron's fighter tactics against the Luftwaffe. In essence, the tactics came down to getting in close, firing in short bursts, using height for advantage, turning to face an attack, maintaining high cover and hitting hard and quickly before getting out.
The Dieppe Raid or Operation Jubilee was the only occasion that all three Eagle Squadrons saw action operating together.

121 Squadron was formed in the light bomber role at Narborough on 1st January, 1918, equipped with DH9s, it did not become operational and disbanded on 17th August 1918.  The squadron reformed at Bracebridge Heath near Lincoln on 14th October 1918 to operate the DH10, but the Armistice a month later brought plans for its deployment to an end and it disbanded again in November without having received any aircraft.

Following the success of the first 'Eagle' squadron, No. 71, and the increasing numbers of Americans coming forward to join the RAF, it was decided to form a second such unit.  As a result No. 121 was formed at Kirton-in-Lindsey on 14th May, 1941, equipped with Hurricanes.  These were flown on defensive patrols in the North of England until October, when Spitfires arrived.  These were taken south to North Weald in December, 1941, from where the squadron took part in the full range of offensive operations and defensive duties being carried out by Fighter Command at that time, but on 29th September, 1942, the squadron, together with the other two (71 and 133) 'Eagle' squadrons were transferred to the US Army Air Force to become the 4th Fighter Group, with 121 becoming the 335th Fighter Squadron.