[1] The design was approved in 1936 as a long range "cruiser submarine" with a heavy torpedo and gun armament.

[2] They were a significantly improved version of the Pravda class and overcame most of their shortcomings (Conway's Fighting Ships).

Yakubov and Worth state that these were the most successful Soviet submarines of the World War II era, with high speed and good seakeeping.

They had amenities such as a bunk for every sailor, small cabins for each officer, showers, electric heaters (this class was designed to operate primarily in the Arctic), and an electric galley.

American naval constructors inspected K-21 in 1944, and thought the design to be workmanlike but technically inferior to contemporary American boats [citation needed] such as the Gato-class submarines An improved design, the KU class, which was to be of welded construction was in planning in 1941.