240 Squadron of the Royal Air Force was formed at RAF Calshot on 20 August 1918 to provide anti-submarine protection, using its Short 184s seaplanes and Felixstowe F2A flying boats.

It then moved to India in March 1942 where it flew anti-shipping and submarine patrols from Red Hills Lake, where it was disbanded on 1 July 1945.

[6] The squadron began converting to Short Sunderland Mk.Vs from 01 August 1945 before moving to Ceylon in January 1946, where it disbanded on 31 March 1946 at RAF Koggala.

The squadron moved to Northern Ireland in June 1952, where it disbanded on 1 November 1958 at RAF Ballykelly by being renumbered to 203 sqn.

The squadron was equipped with three Thor Intermediate range ballistic missiles, and stationed at RAF Breighton.