British B-25B Mitchell Mk I FK165, June 1942
In the middle of 1941, 23 B-25Bs were renamed as Mitchell Mk I and were supplied to the RAF. The majority of these aircraft were assigned to the 111 OTU stationed in the Bahamas. This specific aircraft, FK165 (previously known as B-25B, 40-2339), was assessed by the A&AEE at Boscombe Down in June 1942. On May 29, 1943, the aircraft was removed from service.
The B-25B, which was the 65th Mitchell built, featured a Bendix L-type power-operated turret installed in the rear dorsal position, a notable difference from previous models. The new turret housed a pair of 12.7mm machine guns, along with a second pair located in a Bendix remotely controlled turret positioned ventrally. When not in use, the barrels of the machine guns could be retracted and neatly fitted into a pair of slots located beneath the fuselage. However, this ventral turret was not popular with combat units in the field due to its vulnerability to debris damage, and it was typically removed to lighten the B-25B by 272kg (600lb) and make space for an additional fuel tank instead. Further armament changes were more subtle, such as the removal of a spare 7.62mm machine gun from the waist position, and reducing the size of the waist hatches and Plexiglas windows. The 7.62mm gun in the nose was retained, but the 12.7mm gun in the tail was removed, and the armor plate was replaced by a clear-vision tail cone, which was only used for observation. By rearranging the tail, some weight was traded off against the two Bendix turrets, but the overall weight increased to 9072kg (20,000lb) empty and 12,910kg (28,460lb) with a full load. NAA was able to fulfill the original contract for 184 aircraft before the end of the B-25B production line, leaving time for preparation of the next variant.