U 31 was a class of U-boats built during World War I by the Kaiserliche Marine. U 31 U-boats carried 6 torpedoes and were originally armed with one deck gun, which was replaced in 1916/17 by a deck gun. They carried a crew of 35 and had a cruising range of around . Between 1912 and 1915 11 were built on Germaniawerft in Kiel, amongst these top-three-scoring with the famous Lothar von Arnauld de la Perière as commander, with Walter Forstmann and with Max Valentiner. Together these U-boats sunk more than .