Royal Flying Corps 100 Squadron was formed 23 February 1917 as the first night bombing squadron of RFC. They operated Royal Aircraft Factory F.E.2Bs. Manfred von Richthofen's diary records their raid on his airfield of Douai, on 5 April 1917. In August 1918 it was re-equipped with Handley Page O/400.
The Second World War saw the unit at Seletar, Singapore, with obsolete Vickers Vildebeest. It took part to the defense of Singapore against the Japanese, taking heavy losses. After Singapore was lost, some of its personneel was evaquated to Austrailia and joined 100 Squadron RAAF. On 15 December 1942, 100 Squadron reformed at Grimsby, Lincolnshire, and was equipped with 16 Avro Lancaster MkIII. It started operations on 4 March 1943 against St Nazaire, attacking a U-Boat base. It then took part to all main operations against Europe, including Berlin's bombings and the preparation of the Normandy landings on 5 June 1944.
On 1st of April 1945 the unit was moved to Elsham Wolds, Lincolnshire.