|Ordered:||14 October 1941|
|Builder:||Deutsche Werft AG, Hamburg|
|Laid down:||31 March 1943|
|Launched:||18 November 1943|
|Commissioned:||9 February 1944|
|Fate:||transferred as war booty to the Soviet Union|
|Acquired:||5 November 1945|
|Commissioned:||13 February 1946|
|Decommissioned:||29 December 1955|
|Stricken:||31 January 1968|
|Fate:||broken up for scrap in 1968|
|Class and type:||Type IXC/40 submarine|
|Height:||9.60 m (31 ft 6 in)|
|Draught:||4.67 m (15 ft 4 in)|
|Test depth:||230 m (750 ft)|
|Complement:||4 officers, 44 enlisted|
|Identification codes:||M 43 319|
German Type IXC/40 submarines were slightly larger than the original Type IXCs. U-1231 had a displacement of 1,144 tonnes (1,126 long tons) when at the surface and 1,257 tonnes (1,237 long tons) while submerged. The U-boat had a total length of 76.76 m (251 ft 10 in), a pressure hull length of 58.75 m (192 ft 9 in), a beam of 6.86 m (22 ft 6 in), a height of 9.60 m (31 ft 6 in), and a draught of 4.67 m (15 ft 4 in). The submarine was powered by two MAN M 9 V 40/46 supercharged four-stroke, nine-cylinder diesel engines producing a total of 4,400 metric horsepower (3,240 kW; 4,340 shp) for use while surfaced, two Siemens-Schuckert 2 GU 345/34 double-acting electric motors producing a total of 1,000 shaft horsepower (1,010 PS; 750 kW) for use while submerged. She had two shafts and two 1.92 m (6 ft) propellers. The boat was capable of operating at depths of up to 230 metres (750 ft).
The submarine had a maximum surface speed of 18.3 knots (33.9 km/h; 21.1 mph) and a maximum submerged speed of 7.3 knots (13.5 km/h; 8.4 mph). When submerged, the boat could operate for 63 nautical miles (117 km; 72 mi) at 4 knots (7.4 km/h; 4.6 mph); when surfaced, she could travel 13,850 nautical miles (25,650 km; 15,940 mi) at 10 knots (19 km/h; 12 mph). U-1231 was fitted with six 53.3 cm (21 in) torpedo tubes (four fitted at the bow and two at the stern), 22 torpedoes, one 10.5 cm (4.13 in) SK C/32 naval gun, 180 rounds, and a 3.7 cm (1.5 in) Flak M42 as well as two twin 2 cm (0.79 in) C/30 anti-aircraft guns. The boat had a complement of forty-eight.
U-1231 was ordered in October 1941 from Deutsche Werft AG Weser in Hamburg-Finkenwerder under the yard number 394. Her keel was laid down on 31 March 1943 and was launched on 18 November 1943. About three months later she was commissioned into service under the command of Kapitän zur See Hermann Lessing (Crew 21) in the 31st U-boat Flotilla.
After completing training and work-up for deployment U-1231 was transferred to the 33rd U-boat Flotilla for front-line service on 1 October 1944. The U-boat left Bergen on 18 October 1944 for the first war patrol operating unsuccessfully against Allied shipping in the North Atlantic and off the coast of Canada. After returning to Flensburg on 5 February 1945, Lessing was relieved as commander by Oberleutnant zur See Helmut Winke (Crew X/39). In April 1945 'U-1231 left Kiel for the North Atlantic, again operating without success. After the German surrender, Winke took U-1231 to Dundee, from where the U-boat was transferred to Lisahally.
In November 1945, U-1231 was allocated to the Soviet Union as war booty and was transferred to Libau via Copenhagen between 24 November and 5 December 1945. Renamed N-26 the U-boat was commissioned into the Soviet Navy and served with the Red Banner Baltic Fleet. On 29 December 1955, having been re-designated B-26, the U-boat was decommissioned and placed into reserve and used for training purposes. Struck from the list on 31 January 1968 and sold for scrap the U-boat was later broken up in Riga.