As one of the first ten German Type VII submarines later designated as Type VIIA submarines, U-31 had a displacement of when at the surface and while submerged. She had a total length of, a pressure hull length of, a beam of, a height of, and a draught of. The submarine was powered by two MAN M 6 V 40/46 four-stroke, six-cylinder diesel engines producing a total of for use while surfaced, two BBC GG UB 720/8double-acting electric motors producing a total of for use while submerged. She had two shafts and two propellers. The boat was capable of operating at depths of up to. The submarine had a maximum surface speed of and a maximum submerged speed of. When submerged, the boat could operate for at ; when surfaced, she could travel at. U-31 was fitted with five torpedo tubes, 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.
Service history
During her career U-31 was involved in seven war patrols.
After her quick return to Germany, U-31 became one of three Type VII reserve boats, going to sea again when Karl Dönitz ordered a redeployment of the U-boat force on 8 September. Ordered along with to save time by directly proceeding to the Atlantic via the English Channel, she attacked the first convoy of World War II, OB 4 on 16 September 1939, sinking the British steamer SS Aviemore. Habekost had found the convoy the day before and per orders reported the convoy's location, course and speed to Dönitz. Excited by this first reliable convoy report, he ordered all available boats to converge and attack the convoy. U-31 maneuvered into attack position and in the nighttime hours made an attack. Thinking he had sunk two ships, Habekost had however suffered torpedo failure, and thus only the 4,000 ton Aviemore was actually sunk out of the convoy by U-31. U-31 would later sink the slightly larger Hazelside, of 4,646 tons, on 24 September before concluding the patrol and returning to Wilhelmshaven on 2 October 1939.
First sinking
On 11 March 1940 U-31 was sunk in the Schillig Roads near buoy 12 by four bombs from a Bristol Blenheim, O of No. 82 Squadron RAF, with the loss of 58 lives. The U-boat had been on trials and carried eleven workers from the shipyard and two assistants to the flotilla engineer in addition to her regular complement. The U-boat was raised later that month, repaired and returned to service on 30 July 1940 with Kptlt. Prellberg in command.
Second sinking
U-31 was sunk again on 2 November 1940, north-west of Ireland, by depth charges from the British destroyer, which picked up 44 survivors, from the crew of 46. In U-31s entire career she sank eleven ships, totalling, and one auxiliary warship of. A mine laid by U-31 damaged the British battleship of 33,950 tons.
