During the 1930s, the construction of overseas patrol submarines waned considerably. However, in 1934/35 the Admiralty decided to build a new ocean-going boat to replace the Oberon, Parthian and Rainbow Classes that had not lived up to expectations.
Requirements for the new class (defined as Patrol Submarines) demanded that they had a strong armament and a patrol duration of at least 42 days. Restricted by the limitations imposed by the London Naval Treaty that allowed a total of only 16 500 tons of new construction submarines, the class was designed to have a displacement of about 1000 tons so that a sufficient number could be built. The first-of-class, approved in the 1935 Programme, was built at Vickers and entered service in December 1938 under the name of Triton.
