Somewhat ironically, considering that the aircraft had first been developed as an unarmed bomber, the most numerous variant of the Mosquito was the FB Mk VI fighter bomber, which combined the eight guns of the F Mk II with the capacity to carry a useful bomb load.
Work first began on the FB Mk VI in the summer of 1941. The first prototype flew on 1 June 1942, and production aircraft began to appear in February 1943. Early aircraft were powered by 1,460 hp Merlin 21 or 23 engines, later replaced by the 1,635 hp Merlin 25.
The FB Mk VI was armed with four .303in machine guns and four 20mm cannon, just as had been planned for the day fighter version. It could carry two 500lb bombs in the rear half of its bomb bay (the front half was used by the cannons). Additionally the Mk VI had two wing mounting points that allowed it to carry either 50 gallon drop tanks, or two more 500lb bombs, for a total bomb load of 2,000lbs. Fully armed the FB Mk VI had an effective range of over 1000 miles.
Late in 1944 the Mosquito FB Mk VI was used to carry up to eight rocket projectiles. The first Mosquito attack with RPs was carried out in October 1944.
The FB Mk VI entered service with No. 418 Squadron, which received its first aircraft on 11 May 1943. Eventually it equipped 26 RAF squadrons, seeing service over Europe and the Far East as well as from bases on Malta. It was also used by Coastal Command on anti-shipping duties. The FB Mk VI carried out some of the most daring Mosquito raids of the war, amongst them the famous attack on Amiens Prison on 18 February 1944.