121 Squadron Royal Flying Corps (Royal Air Force from 1 April 1918) was formed at Narborough on 1 January 1918 as a day bomber squadron to operate the Airco DH.9.

Equipped with the Hawker Hurricane, the squadron then converted to the Supermarine Spitfire and moved south to RAF North Weald to begin operations on channel sweeps and Rhubarb operations.

On 15 November 1941 the squadron claimed its first enemy aircraft destroyed.

The squadron then upgraded to cannon-armed Spitfire VBs and carried out offensive operations over the channel and providing bomber escorts.

The new squadron became part of the 4th Fighter Group of the Eighth Air Force.