U-31 was fitted with five  torpedo tubes (four fitted at the bow and one at the stern), eleven torpedoes, one  SK C/35 naval gun, 220 rounds, and an anti-aircraft gun. The boat had a complement of between forty-four and sixty.
U-31 was equipped with five torpedo tubes (four at the bow and one at the stern), eleven torpedoes, one SK C/35 naval gun, 220 rounds, and an anti-aircraft gun. The vessel had a crew of between forty-four and sixty.