The Fairmile B motor launch was a type of motor launch built by British boat-builder Fairmile Marine during the Second World War for the Royal Navy for coastal operations. Altogether approximately 650 boats were built between 1940 and 1945. Like the A Type, the B Type were initially intended as submarine chasers, so the boats were fitted with ASDIC (sonar) as standard. Their main armament initially reflected their anti-submarine focus, with 12 depth charges, a single QF 3-pounder Hotchkiss gun aft, and one set of twin 0.303-in machine guns.
As the war moved on, the vessels were adapted to other roles and the armament was modified and upgraded such as the replacement of the 3 pounder with one or more 20 mm Oerlikon cannon and removal of the ASDIC dome for more clearance as minesweepers. Some boats were configured as motor torpedo boats.
During the Siege of Malta, they were used to sweep a narrow channel ahead of heavier minesweepers which widened the channel. The heavier minesweepers were initially the remnants of the Malta trawler force, then fleet minesweepers that arrived with a convoy from Gibraltar. The launches were able to pass over the mines whereas many trawler losses had been caused by the leading ship hitting a mine.
A number served in the St Nazaire Raid as assault transports, but their light construction meant that they suffered heavily; 12 B Motor Launches were lost in the action, out of 16 deployed. During the Normandy landings a number of MLs were designated as navigation launches. These motor launches guided the landed craft onto the correct beaches. For this task the craft were fitted with splinter mats at the front for added protection. The main armament - the 3-pounder gun - was moved to the bow, an Oerlikon 20 mm cannon was fitted amidships and a Bofors 40 mm gun was installed at the stern. Smoke canister apparatus was installed at the rear of the craft and the number of depth charges was reduced
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