Driven by the news of German U-cruisers with 5.9-inch guns, the Committee on Submarine Development decided to construct submarine monitors - the M Class - with 12-inch guns.
Four K Class submarines, K18 to K21, had been ordered in February 1916 but, when Vickers received an order to build a boat to the new design, K18 was remodelled and became M1. M2 (ex K19) was ordered from Vickers in May 1916 and two more, M3 (ex K20) and M4 (ex K21), from Armstrong Whitworth in August 1916.
Of the three M Class submarines built (M4 was scrapped before completion), only M1 was completed before the end of the war, though she was never used in action because it was thought that if she was copied by the enemy Britain was likely to suffer more from the use of the 12-inch gun than Germany. Why this was never thought of before construction remains a mystery. M2 and M3 were completed in 1920.
