Type U 31 was a class of U-boats built during World War I by the Kaiserliche Marine.
German Type U 31 submarines were double-hulled ocean-going submarines similar to Type 23 and Type 27 subs in dimensions and differed only slightly in propulsion and speed. They were considered very good high sea boats with average manoeuvrability and good surface steering.
Type U-31s had an overall length of , their pressure hulls were long. The boats' beam was (o/a), while the pressure hull measured . Type 31s had a draught of with a total height of . The boats displaced a total of ; when surfaced and when submerged.
Type U-31s were fitted with two Germania 6-cylinder two-stroke diesel engines with a total of for use on the surface and two Siemens-Schuckert double-acting electric motors with a total of for underwater use. These engines powered two shafts each with a propeller, which gave the boats a top surface speed of , and when submerged. Cruising range was at on the surface. Constructional diving depth was .
The U-boats were armed with four torpedo tubes, two fitted in the bow and two in the stern, and carried 6 torpedoes. Most boats received one or two SK L/30 deck guns, which were later replaced with SK L/45 guns on some. The boats' complement was 4 officers and 31 enlisted.
Between 1912 and 1915 eleven were built on Germaniawerft in Kiel. Problems with the timely delivery of their diesel engines led to delays up to eleven months, and they were commissioned between September 1913 and February 1915.
The top three-scoring U-boats were Type 31 U-boats : with the famous Lothar von Arnauld de la Perière as commander, with Walter Forstmann and with Max Valentiner.
Later admiral and head of the Abwehr from 1935 to 1944 Wilhelm Canaris also served as commander on two different Type U 31 U-boats. He first took over from Max Valentiner on U-38 and later on .
There were eleven Type U 31 submarines commissioned into the Kaiserliche Marine.