The 335th was constituted on 22 August 1942 as an incorporation of the No. 121 (Eagle) Squadron of the Royal Air Force, formed on 14 May 1941 as the second of three Eagle Squadrons of the Royal Air Force. These squadrons were composed of American volunteers, recruited by the RAF as a result of the heavy loss of pilots during the Battle of Britain in 1940; the volunteers were ineligible to join the USAAF. In this capacity, the squadron operated Supermarine Spitfires and Hawker Hurricanes.