One of the ocean-going submarines of the S-class (Srednyaya, IX-bis series) whose initial design was developed by German engineers under the contract of the U.S.S.R. As the series was being built, many improvements were made to the project. In 1954 and 1955, four boats of this type, including S-25, renamed Xin Zhong Guo 14 (New China No. 14), were transferred to the PRC and became the basis of that countrys submarine forces.