No. 504 Squadron was one of the Special Reserve Squadrons of the Auxiliary Air Force, and today is a reserve force of the RAF Regiment. It was integrated into the AAF proper in 1936. Based at RAF Cottesmore, Rutland, 504 Squadron used a variety of light bombers before being re-tasked to fighters with the Hawker Hurricane in 1939. It subsequently became a Fighter Squadron. Currently No. 504 Squadron no longer has a flying role, but as part of No 85 Expeditionary Logistics Wing of the RAF A4 Force.

The Royal Auxiliary Air Force (RAuxAF), formerly the Auxiliary Air Force (AAF), together with the Air Force Reserve, is a component of His Majesty's Reserve Air Forces. It provides a primary reinforcement capability for the regular service, and consists of paid volunteers who give up some of their weekends, evenings and holidays to train at one of a number of squadrons around the United Kingdom. Its current mission is to provide trained personnel in support of the regular RAF.