U-20 was a Type IIB submarine — a design intended for coastal patrol and minelaying rather than the long-range Atlantic campaigns associated with the larger Type VII and Type IX boats. Her keel was laid down at Germaniawerft in Kiel on 1 August 1935, and she was commissioned on 1 February 1936. At 279 tonnes surfaced and 328 tonnes submerged, she was a compact vessel: 42.7 metres overall, powered by twin MWM diesel engines producing 700 horsepower for surface running and Siemens-Schuckert electric motors for submerged operations. She could make 13 knots on the surface, 7 submerged, and cruise up to 3,800 nautical miles at 8 knots — useful range for North Sea operations. Her armament was three forward torpedo tubes and room for twelve mines, plus a 2-centimetre anti-aircraft gun. Twenty-five men crewed her.

U-20's first three patrols in 1939 and early 1940 took her to the North Sea and the British east coast, laying mines and observing enemy movements. On her fourth patrol, she sank the vessel Magnus approximately 40 nautical miles east-northeast of Peterhead, Scotland. The ship went down in 90 seconds; there was only one survivor. U-20 went on to sink Ionian and Willowpool with mines laid in November 1940, and sank Sylvia northeast of Aberdeen on 13 October 1940. The grim arithmetic of submarine warfare: ships sunk swiftly, crews lost, the numbers tallied in admiralty records on both sides. U-20 was then transferred to a training role as a school boat on 1 May 1941 before the Kriegsmarine hatched its plan to open a new front in the Black Sea.

Moving a submarine overland — or rather, over river — required dismantling it. U-20 was cut into sections that could be loaded onto river barges and floated from Germany down through occupied Europe to Romania. At the Galați shipyard on the Danube, Romanian engineers reassembled her and relaunched her into the river, which emptied into the Black Sea not far downstream. She was then assigned to the 30th U-boat Flotilla, the small force Germany maintained in the Black Sea to contest Soviet naval superiority. It was a completely enclosed sea, with no connection to any ocean. The submarines that entered it could never leave through the Bosphorus — Turkish neutrality and Allied pressure ensured that — and the Soviets controlled the eastern and northern coasts. The Black Sea became a trap as much as a theater.

U-20's Black Sea patrols, numbered ninth through sixteenth in her overall war record, were a mixed experience. Her first patrol in the new theater nearly ended in disaster when she attacked a Soviet submarine chaser and the vessel responded with eight depth charges; U-20 stayed submerged for four hours and returned to base with mechanical failures. Later patrols between the Romanian port of Constanța and Sevastopol — a city that changed hands brutally between German and Soviet forces during 1942 and 1944 — were steadier. She sank the Soviet vessel Vaijan Kutur'e on 16 January 1944 off Cape Anakria, and on 19 June 1944 sank Pestel off Trabzon on the Turkish coast. The Soviets noted that Pestel went down in Turkish territorial waters. On 26 June 1944 she destroyed the Soviet landing craft DB-26 with gunfire and demolition charges. By that point the war's outcome was not in doubt, and the Black Sea flotilla's days were numbered.

On 10 September 1944, as Soviet forces swept through Romania and the strategic situation collapsed, U-20 was scuttled off the Turkish coast of the Black Sea. Her crew survived. The boat settled in roughly 80 feet of water — shallow enough, as it turned out, to be found. For decades she lay unknown on the seabed. Then in 2008, the Sunday Telegraph reported that U-20 had been located by Selçuk Kolay, a Turkish marine engineer, off the coast of the city of Karasu — though Turkish Navy divers had first found the wreck during an exercise in July 1994. The wreck sits at approximately 41.17°N, 30.78°E, in the same stretch of Turkish Black Sea coast where her World War I predecessor SM UC-13 ran aground nearly three decades earlier. Two German submarines, two different wars, the same sea floor.