The improved Defiant Mk II model was fitted with the AI Mk. IV radar and a Merlin XX engine, increasing the aircraft's performance, particularly at night time. In September 1941, 264 Squadron became the first to receive the Defiant Mk II, bringing them into operational use by mid-September. The principal users of the Mk II night fighter were 96, 151, and 262 Squadrons. As the radar-equipped Defiants began filtering through to operational squadrons, the "Luftwaffe"'s bombing campaign petered out as German forces had become heavily engaged on the Eastern Front as they embarked upon the Operation Barbarossa the invasion of the Soviet Union.