[Note 1] In 13 patrols UC-31 was credited with sinking 38 ships, either by torpedo or by mines laid.

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).

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.