During the First World War, following a suggestion from three junior officers of the Harwich destroyer force that small motor boats carrying a torpedo might be capable of travelling over the protective minefields and attacking ships of the Imperial German Navy at anchor in their bases, the Admiralty gave tentative approval to the idea and, in the summer of 1915, produced a Staff Requirement requesting designs for a Coastal Motor Boat for service in the North Sea.

The speed of the boat when fully loaded was to be at least 30 knots (56 km/h) and sufficient fuel was to be carried to give a considerable radius of action.

A number of these boats were built and had a distinguished service history, but in hindsight they were considered to be too small to be ideal, particularly in how their payload was limited to a single 18-inch torpedo.

There is no record of a CMB ever being hit by its own torpedo, but in one instance the firing pistol was triggered prematurely and the crew had a tense 20 minutes close to the enemy whilst reloading it.

At least two unexplained losses due to fires in port are thought to have been caused by a build-up of petrol vapour igniting.

Five were cancelled; of the remainder, 3 survived the Second World War,[6] with CMB 103 MT, built by Camper and Nicholsons in 1920, preserved as a museum ship.

[11] Sixteen Coastal Motor Boats were lost during the war[12] In 1919 the British operated against Soviet Red forces in the Baltic.

Coastal Motor Boats were initially used for secret duties to move agents in and out of Russia from a base in Finland.

[13] But in June 1919, the commander of the force, Lieutenant Augustus Agar, took two CMBs in a raid on Bolshevik ships in Kronstadt.

[5][14] In August, a larger operation supported with aircraft managed to damage one battleship and sink a depot ship.

[20] The hull of CMB 4 in which Augustus Agar won his VC for the attack on Kronstadt naval base in 1919 and sank the cruiser Oleg was, for many years, at the Vosper Thornycroft works on Platt's Eyot on the Thames near Kingston.

When these works closed it was restored and can now be seen in Boathouse 4 at Portsmouth Historic Dockyard where it is on loan from the Imperial War Museum, Duxford with details of these boats and the action.

[23] MTB 331, owned by Hampshire County Council and on-loan to the British Military Powerboat Trust (BMPT) at Marchwood, is the sole surviving 55-foot CMB.