The Bristol Blenheim was used by both Bomber and Fighter Commands.
Some two hundred 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 in September 1938.
By 1939, at least seven squadrons were operating these twin-engined fighters and within a few months, some sixty squadrons had experience of 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 decided that the Mk IF would be relegated mainly to night fighter duties where No.
23 Squadron RAF, which had already operated the type under nighttime conditions, had better success.

In the German night-bombing raid on London on 18 June 1940, Blenheims accounted for five German bombers, thus proving that they were better-suited for night fighting.
In July, No.
600 Squadron, by then based at RAF Manston, had some of its Mk IFs equipped with AI Mk III radar.
With this radar equipment, a Blenheim from the Fighter Interception Unit (FIU) at RAF Ford achieved the first success on the night of 2–3 July 1940, accounting for a Dornier Do 17 bomber.
More successes came, and before long the Blenheim proved itself invaluable as a night fighter.
Gradually, with the introduction of the Bristol Beaufighter in 1940–1941, the Blenheim was supplanted by its faster, better-armed descendant.
The Bristol Blenheim was used by both Bomber and Fighter Commands. Some two hundred 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 in September 1938. By 1939, at least seven squadrons were operating these twin-engined fighters and within a few months, some sixty squadrons had experience of 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 decided that the Mk IF would be relegated mainly to night fighter duties where No. 23 Squadron RAF, which had already operated the type under nighttime conditions, had better success.
In the German night-bombing raid on London on 18 June 1940, Blenheims accounted for five German bombers, thus proving that they were better-suited for night fighting. In July, No. 600 Squadron, by then based at RAF Manston, had some of its Mk IFs equipped with AI Mk III radar. With this radar equipment, a Blenheim from the Fighter Interception Unit (FIU) at RAF Ford achieved the first success on the night of 2–3 July 1940, accounting for a Dornier Do 17 bomber. More successes came, and before long the Blenheim proved itself invaluable as a night fighter. Gradually, with the introduction of the Bristol Beaufighter in 1940–1941, the Blenheim was supplanted by its faster, better-armed descendant.
Bristol Blenheim