“K-21” was a Soviet diesel-electric cruiser submarine of the Second World War era. She was part of the Northern Fleet of the USSR Navy. She successfully took part in 12 combat campaigns, six minelaying expeditions, ten torpedo attacks and two artillery battles. According to the Soviet command, the submarine sank 17 enemy vessels and ships.

Probably the most famous moment in her service was the attack on the German battleship Tirpitz in the area of Ingøya Island. The Tirpitz was a Bismarck-class battleship that was part of the naval forces of Nazi Germany. She practically did not participate in the fighting, but with her presence in Norway the battleship threatened the Arctic convoys headed to the USSR. In addition, the ship held down significant forces of the British fleet. For more than two and a half years, attempts were made to destroy the battleship Tirpitz.

On July 5, 1942, the battleship, which was part of the squadron under the command of Admiral Otto Schniewind, was attacked by the Soviet submarine under the command of Captain Second Rank Nikolay Lunin. The submarine fired four torpedoes from a long range. Captain Lunin assessed the attack as a success, hearing the sounds of explosions, which he interpreted as accurate hits by the torpedoes. Subsequently, reports in the Soviet press claimed that the battleship was damaged by the torpedoes. However, the German squadron did not notice any torpedo hits.

The modern reconstruction of the incident, based on all available information, shows that the attack did not have even a theoretical chance of success. The torpedoes were launched from a distance greater than their maximum range. Nevertheless, this event occupies an important place in the history of the Soviet Navy, as it represents the only instance of Soviet submariners launching an attack against a heavy enemy warship. Interestingly, historians continue to debate this event. In November 1944, the battleship was rendered inoperable after an air attack with British Tallboy heavy bombs.

After the end of the war, on May 9, 1945, the K-21 submarine was sent to undergo repairs. Later, she took part in oceanographic work in the area of the Novaya Zemlya archipelago. In 1949, the submarine was renamed B-4, and in 1956 it was withdrawn from the fleet. Since 1983, the submarine has been permanently moored at the berth of the city of Severomorsk.