German Type VIIC submarines were preceded by the shorter Type VIIB submarines. U-640 had a displacement of 769t when at the surface and 871t while submerged. She had a total length of 67.1m (220.1feet), a pressure hull length of 50.5m (165.7feet), a beam of 6.2m (20.3feet), a height of 9.6m (31.5feet), and a draught of 4.74m (15.55feet). The submarine was powered by two Germaniawerft F46 four-stroke, six-cylinder supercharged diesel engines producing a total of NaNPS for use while surfaced, two Brown, Boveri & Cie GG UB 720/8 double-acting electric motors producing a total of 750PS for use while submerged. She had two shafts and two 1.23abbr=onNaNabbr=on propellers. The boat was capable of operating at depths of up to .
The submarine had a maximum surface speed of and a maximum submerged speed of . When submerged, the boat could operate for at ; when surfaced, she could travel at . U-640 was fitted with five 53.30NaN0 torpedo tubes (four fitted at the bow and one at the stern), fourteen torpedoes, one 8.82NaN2 SK C/35 naval gun, 220 rounds, and one twin 2sp=usNaNsp=us C/30 anti-aircraft gun. The boat had a complement of between forty-four and sixty.
The boat's career began with training at 5th U-boat Flotilla on 17 September 1942, followed by active service on 1 May 1943 as part of the 6th Flotilla.
In 1 patrol she sank no ships.
U-640 took part in one wolfpack, namely:
There are discrepancies between the fate of U-640 and, which was sunk in the same vicinity around the same time. Paul Kemp's book[1] attributes the sinking of British freighter Aymeric to U-640, and then subsequently sunk by on the 17 May 1943, whereas UBoat.net attributes this success and fate to U-657.
Uboat.net states that U-640 was sunk on 14 May 1943 in the Atlantic Ocean in position 60.5333°N -36°W, by depth charges from a US Catalina of VP-84. All hands were lost.