|Ordered||14 October 1942|
|Builder||Flender Werke, Lübeck|
|Laid down||21 January 1943|
|Launched||27 November 1943|
|Commissioned||20 April 1944|
|Fate||Sunk by a Polish aircraft, southwest of Ireland, April 1945|
|Class and type||Type VIIC/41 submarine|
|Height||9.60 m (31 ft 6 in)|
|Draught||4.74 m (15 ft 7 in)|
|Complement||4 officers, 40–56 enlisted|
|Identification codes:||M 44 228|
She carried out two patrols, but did not sink any ships.
The boat was sunk in April 1945 by a Polish aircraft in the Atlantic Ocean.
German Type VIIC/41 submarines were preceded by the heavier Type VIIC submarines. U-321 had a displacement of 759 tonnes (747 long tons) when at the surface and 860 tonnes (850 long tons) while submerged. She had a total length of 67.10 m (220 ft 2 in), a pressure hull length of 50.50 m (165 ft 8 in), a beam of 6.20 m (20 ft 4 in), a height of 9.60 m (31 ft 6 in), and a draught of 4.74 m (15 ft 7 in). The submarine was powered by two Germaniawerft F46 four-stroke, six-cylinder supercharged diesel engines producing a total of 2,800 to 3,200 metric horsepower (2,060 to 2,350 kW; 2,760 to 3,160 shp) for use while surfaced, two Garbe, Lahmeyer & Co. RP 137/c double-acting electric motors producing a total of 750 metric horsepower (550 kW; 740 shp) for use while submerged. She had two shafts and two 1.23 m (4 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 17.7 knots (32.8 km/h; 20.4 mph) and a maximum submerged speed of 7.6 knots (14.1 km/h; 8.7 mph). When submerged, the boat could operate for 80 nautical miles (150 km; 92 mi) at 4 knots (7.4 km/h; 4.6 mph); when surfaced, she could travel 8,500 nautical miles (15,700 km; 9,800 mi) at 10 knots (19 km/h; 12 mph). U-321 was fitted with five 53.3 cm (21 in) torpedo tubes (four fitted at the bow and one at the stern), fourteen torpedoes, one 8.8 cm (3.46 in) SK C/35 naval gun, (220 rounds), one 3.7 cm (1.5 in) Flak M42 and two 2 cm (0.79 in) C/30 anti-aircraft guns. The boat had a complement of between forty-four and sixty.
The submarine was laid down on 21 January 1943 by the Flender Werke yard at Lübeck as yard number 321, launched on 27 November 1943 and commissioned on 20 January 1944 under the command of Kapitänleutnant Ulrich Drews.
Forty-one men died; there were no survivors.