Late in September 1940 the squadron received its first Bristol Beaufighter, equipped with four 20-mm Hispano-Suiza HS.404 cannon under the nose and improved Mark IV AI radio-location equipment. As one of the few squadrons thus equipped, 604 Squadron provided night defence over the UK during the Blitz from late 1940 until mid May 1941, when most Luftwaffe bomber units departed for involvement in the invasion of Russia. By this time 50 air victories had been claimed by the squadron—fourteen by F/L John Cunningham.