|Laid down:||December 1914|
|Commissioned:||8 January 1916|
|Fate:||Sold, 6 September 1922|
|Class and type:||E class submarine|
|Length:||181 ft (55 m)|
|Beam:||15 ft (4.6 m)|
Like all post-E8 British E-class submarines, E31 had a displacement of 662 tonnes (730 short tons) at the surface and 807 tonnes (890 short tons) while submerged. She had a total length of 180 feet (55 m) and a beam length of 22 feet 8.5 inches (6.922 m). She was powered by two 800 horsepower (600 kW) Vickers eight-cylinder two-stroke diesel engines and two 420 horsepower (310 kW) electric motors. The submarine had a maximum surface speed of 16 knots (30 km/h; 18 mph) and a submerged speed of 10 knots (19 km/h; 12 mph). British E-class submarines had fuel capacities of 50 tonnes (55 short tons) of diesel and ranges of 3,255 miles (5,238 km; 2,829 nmi) when travelling at 10 knots (19 km/h; 12 mph). E31 was capable of operating submerged for five hours when travelling at 5 knots (9.3 km/h; 5.8 mph).
E31 was armed with a 12-pounder QF gun mounted forward of the conning tower. She had five 18 inches (460 mm) torpedo tubes, two in the bow, one either side amidships, and one in the stern; a total of 10 torpedoes were carried.
E-Class submarines had wireless systems with 1 kilowatt (1.3 hp) power ratings; in some submarines, these were later upgraded to 3 kilowatts (4.0 hp) systems by removing a midship torpedo tube. Their maximum design depth was 100 feet (30 m) although in service some reached depths of below 200 feet (61 m). Some submarines contained Fessenden oscillator systems.
HMS E31 was involved in a curious incident when she was operating with the sea plane carrier Engadine in the North Sea in an air raid on the Zeppelin sheds at Tondern on 4 May 1916. HMS E31 surfaced and spotted Zeppelin L 7. She dived to avoid attack. While at periscope depth, it was observed that the Zeppelin was losing altitude as it had been hit by shells from the light cruisers Galatea and Phaeton. HMS E31 surfaced to shoot the Zeppelin down and rescued seven survivors.
- Innes McCartney; Tony Bryan (20 February 2013). British Submarines of World War I. Osprey Publishing. pp. 11–12. ISBN 978-1-4728-0035-0.
- Akerman, P. (1989). Encyclopaedia of British submarines 1901–1955. 149–150. Maritime Books. ISBN 1-904381-05-7 
- "E Class". Chatham Submarines. Retrieved 20 August 2015.
- Hutchinson, Robert (2001). Jane's Submarines: War Beneath the Waves from 1776 to the Present Day. London: HarperCollins. ISBN 978-0-00-710558-8. OCLC 53783010.