Nicknamed "Dog Boats", they were designed to be assembled in kit form mass-produced by the Fairmile organisation and assembled at dozens of small boatbuilding yards around Britain, to combat the known advantages of the German E-boats over previous British coastal craft designs.

633 to 640) were chosen for Mediterranean operations; they were worked up at HMS Bee at Weymouth before proceeding to Milford Haven, where they were formed into a new 32nd MTB Flotilla under the command of Lt. P. E. Stewart Gould; they were fitted with extra temporary fuel tanks bolted to the upper deck (raising their fuel capacity by 3,000 gallons to 8,000 gallons of 100 octane petrol for the long passage to the Mediterranean), and proceeded in groups to Gibraltar in March/April 1943.

697 onwards, the boats were classified as combined "MTB"/"MGB"s. As with the final eight boats of the preceding programme, 24 boats were chosen for Mediterranean operations; they were worked up at HMS Bee at Weymouth before proceeding to Milford Haven, where they were formed into three new flotillas as follows (with initial flotilla commanders in parentheses): As before, they were fitted with extra temporary fuel tanks bolted to the upper deck (raising their fuel capacity by 3,000 gallons to 8,000 gallons of 100 octane petrol for the long passage to the Mediterranean), and proceeded in groups to Gibraltar in March/April 1943.

Unlike the Fairmile B designs (many of which were built overseas), the "Dog Boats" were only produced in component form in Britain.

Some were built for the RAF Marine Branch for use in the long range air-sea rescue role for downed airmen.

By 1944, the MGB designation was largely dropped by the RN and most of the mid-war (earlier model) Type Ds which had survived were reclassified as MTBs even if they lacked torpedo armament.