The Bristol Blenheim was used by both Bomber and Fighter Commands. Some 200 Mk. I bombers were modified into Mk. IF long-range fighters with 600 (Auxiliary Air Force) Squadron based at Hendon, the first squadron to take delivery of these variants in September 1938. By 1939, at least seven squadrons were operating these twin-engined fighters and within a few months some 60 squadrons had transitioned to the type. The Mk. IF proved to be slower and less nimble than expected and by June 1940, daylight Blenheim losses were to cause concern for Fighter Command. It was then decided that the IF would be relegated mainly to night fighter duties where No. 23 Squadron RAF who had already operated the type under night time conditions had better success.