German submarine U-31 was a Type VIIA U-boat of Nazi Germany's Kriegsmarine during World War II. She was laid down on 1 March 1936 as yard number 912, launched on 25 September and commissioned on 28 December 1936.

Design 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 720/8 double-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 (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.

Service history During her career U-31 was involved in seven war patrols.

First patrol U-31 was one of the few ocean-going submarines deployed to the Baltic Sea instead of the Atlantic Ocean on the eve of World War II. Departing Memel under the command of Johannes Habekost on 27 August, this uneventful trip was concluded quickly with the rapid destruction of the Polish Navy and the boat put in at Wilhelmshaven on 2 September.

Second patrol 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,Blair, page 84 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 4,060 GRT Aviemore was actually sunk out of the convoy by U-31. U-31 would later sink the slightly larger Hazelside, of 4,646 GRT, 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 (or 43, sources vary), from the crew of 46.

In U-31s entire career she sank eleven merchant ships, totalling , and two auxiliary warships of . A mine laid by U-31 damaged the British battleship of 33,950 tons.

Summary of raiding history DateName of ShipNationalityTonnageMerchant ship tonnages are in gross register tons. Military vessels are listed by tons displacement.Fate16 September 1939Aviemore4,060Sunk24 September 1939Hazelside4,646Sunk1 December 1939Arcturus1,277Sunk3 December 1939Ove Toft2,135Sunk4 December 1939Gimle1,271Sunk4 December 193933,950Damaged (mine)4 December 1939Primula1,024Sunk6 December 1939Agu1,575Sunk6 December 1939Vinga1,974Sunk23 December 1939HMS Glen Albyn82Sunk (mine)23 December 1939HMS Promotive78Sunk (mine)22 September 1940Union Jack81Sunk27 September 1940Vestvard4,319Sunk29 October 1940Matina5,389Sunk

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