Even though marine diesel engines of the period were incapable of propelling submarines as fast as surface craft, the Admiralty still wished, perhaps misguidedly, to build Fleet Submarines, and continued to discuss, at length, the functions and requirements of such vessels. In 1928, the culmination of the removal from service of the K Class submarines and the demand by Rear-Admiral (Submarines) for 'submarines which, in an ocean war, should be capable of operating with the Fleet' led to the development of the River Class Fleet Submarine.
Submitted to the Board in June 1929, the River Class were conventional submarines, long, well streamlined and without any large guns or other unusual features. All three vessels of the class were built at Barrow - the first-of-class Thames completing in September 1931, and her slightly larger sister ships, Severn
