Following the outbreak of the Second World War, Britain was faced with the urgent need to equip its fleet with ships capable of combatting German submarines. Although Fairmile A motorboats had already been ordered by that time, the design of new ones continued. A number of shortcomings of the Fairmile A also became apparent during construction, including poor seaworthiness, insufficient range and limited rearmament capabilities. Luckily, by mid-1939, the British Admiralty already had prepared a design for a new boat that were to be more successful than the Fairmile A. In particular, it had a rounded rather than keeled hull, which promised significantly greater seaworthiness. The refinement and production of this design were entrusted to Fairmile Marine, and the boat was designated the Fairmile B.

Between 1940 and 1945, a total of around 650 boats of this type were produced. One of these was the ML345, a late-production boat which entered service on 30 March 1942. The main difference of the later boats was their reinforced armament, which included additional 20 mm guns. During the war, the boat was used in the North Sea. After surviving the war, it was decommissioned in 1946 and sold. Its subsequent fate is unknown.

The Fairmile B (ML345) was introduced in Update 1.83 "Masters of the Sea". The boat has effective armament and a spacious hull, but a small crew.