After conferences between seamen, submarine commanders and naval engineers it was decided to install rather powerful torpedo and artillery together with mine weapons. Soviet naval command approved this project (II series) to be universal long-range minelayer/attack sub, intended to install mine defences onto fairways and enemy naval activities routes, to inflict attacks upon enemy warships and transport ships with a help of torpedoes and artillery. Mines were carried inside the inner hull in two long tubes of special design with hatches aft the boat. 20 mines were secured in dry condition inside this tubes until exact installation. First series of L class, built in 1929-30 (type II), had considerably long time of submerging (about 3 min.).

Latter submarines (type XI, XIII and XIII-1938) had many improvements in equipment, weapons and range. They became really universal boats, fighting on all wartime theatres. Most success in score was achieved by minefields of this boats.