The origin of this project is rather interesting. It is a result of international cooperation between Germany and USSR in the beginning of 1930s.

The USSR S-type subs (USSR IX and IX-bis series) were born from E-1 prototype, that was laid down initially for Spain navy in Cartagena by IvS (Dutch subsidiary of Deschimag AG Weser) in 1930.

As the order was later cancelled, Deschimag had to search for new clients. Several countries including USSR were invited to get acquainted with E-1. In November 1932 three Soviet naval engineers inspected the boat and suggested purchasing line-drawings and documentation for later buildings at Soviet yards. The boat was considered to match the requirements of Soviet shipbuilding program partially, so, Deschimag had to work out the corrected project.

In May 1933 seven leading Soviet naval designers and engineers made a business-trip to Bremen and Karhatena to perform sea tests of E-1 and work on suitable approach to further changes of E-1 project for German engineers taking in mind exact technical requirements of USSR navy. The new project was ready in the end of 1933 and named E-2 (USSR IX series) with enlargement of displacement, artillery and range. Three submarines of IX series were build in Leningrad and partially utilized German machinery. The majority of S-type submarines, however, belonged to IX-bis series, which had been developed taking into consideration just domestic parts and machinery supplies. They were built in Leningrad, Gorki and Nikolayev.

As for technical features of the project, which were decided before WWII, Soviet navy had to fight onto closed war theatres, close to enemy shore and defences, so, long range and other qualities of ocean-going submarines of mid-class could not be achieved as subs had to stay submerged most of time and waste fuel for frequent charging. Nevertheless, under some evaluations, S-type submarines inflicted the most harm to enemy during war.

As for IvS (Deschimag AG Weser) - they used E-1 and E-2 experience as a base for IA series boats, who, in turn, were mothers for famous IX-type Kriegsmarine subs. You should not mix USSR IX series and German IX type boats, nevertheless, they have the same relatives.