SM UC-31 was a German Type UC II minelaying submarine or U-boat in the German Imperial Navy (German: Kaiserliche Marine) during World War I. The U-boat was ordered on 29 August 1915 and was launched on 7 August 1916. She was commissioned into the German Imperial Navy on 2 September 1916 as SM UC-31.^{[Note 1]} In 13 patrols UC-31 was credited with sinking 38 ships, either by torpedo or by mines laid. UC-31 was surrendered on 26 November 1918 and broken up at Canning Town in 1922.^[1]

A Type UC II submarine, UC-31 had a displacement of 400 tonnes (390 long tons) when at the surface and 480 tonnes (470 long tons) while submerged. She had a length overall of 49.45 m (162 ft 3 in), a beam of 5.22 m (17 ft 2 in), and a draught of 3.68 m (12 ft 1 in). The submarine was powered by two six-cylinder four-stroke diesel engines each producing 250 metric horsepower (180 kW; 250 shp) (a total of 500 metric horsepower (370 kW; 490 shp)), two electric motors producing 460 metric horsepower (340 kW; 450 shp), and two propeller shafts. She had a dive time of 48 seconds and was capable of operating at a depth of 50 metres (160 ft).^[3]

The submarine had a maximum surface speed of 11.6 knots (21.5 km/h; 13.3 mph) and a submerged speed of 6.7 knots (12.4 km/h; 7.7 mph). When submerged, she could operate for 53 nautical miles (98 km; 61 mi) at 4 knots (7.4 km/h; 4.6 mph); when surfaced, she could travel 10,040 nautical miles (18,590 km; 11,550 mi) at 7 knots (13 km/h; 8.1 mph). UC-31 was fitted with six 100 centimetres (39 in) mine tubes, eighteen UC 200 mines, three 50 centimetres (20 in) torpedo tubes (one on the stern and two on the bow), seven torpedoes, and one 8.8 cm (3.5 in) Uk L/30 deck gun. Her complement was twenty-six crew members.