The Defiant joined the No. 264 Squadron in December 1939.
Defiants were used as day fighters, night fighters, target towers, air sea rescue, radar calibration, and radar countermeasures.
The first action by six Defiants was in May 1940, and five of these were lost.
On a single day in the month of May 1940 Defiant crews claimed to have shot down 18 German planes with a loss to themselves.
The Defiants had some success during the evacuation from Dunkirk. The Germans were initially surprised by the turret as the Defiant had an appearance as a normal fighter.
Battle of Britain
The Nos 141 and 264 Squadrons suffered horrendous Defiant losses to German pilots who were now used to the Defiant.
Withdrawn to the Night
In August 1940 the Defiant was withdrawn from day operations.
The Defiants were then moved into the night fighter role with the Mk I flying "blind" at night. Thirteen night fighter squadrons were to be equipped with the Defiant.
The No 307 Squadron was manned by Polish pilots and was the first to be used for night fighting in September 1940. In November 1940 the No 255 and No 256 Squadrons were assigned as night fighters. In December 1940 the No 151 Squadron became night fighters. They became operational in 1941 and had AI Mk VI radars installed when they became available.
In January 1941 the No 85 Squadron flew three operations before being converted to Havocs.
No 515 Squadron was used to jam German radar starting in 1942.
The Defiant was relegated to target towing, air-sea rescue, radar calibration, and electronic countermeasures (Mandrel jamming system) after 1941.