The unique feature of the class design was a 12-inch (305 mm) gun mounted in a casemate forward of the conning tower.

Due to the limitations imposed on submarine armament by the Washington Naval Conference, M2 and M3 had their guns removed.

They were initially intended as coastal bombardment vessels, submarine monitors, but their role had been changed before detailed design begun.

A 12-inch gun fired at relatively short range would have a flat trajectory simplifying aiming, and few ships would be expected to survive a single hit.

She was captained during her sea trials by experienced submariner Commander Max Horton after his return from the Baltic, and was later lost with all hands while on exercise in the English Channel near Start Point in Devon after a collision with a Swedish collier, SS Vidar, on 12 November 1925.

Later that year the wreck was visited again by Richard Larn and a BBC TV documentary crew, and the resulting film was aired in March 2000.