From April to June 1940, the crew of
U-99 were under training, based at Kiel and St. Nazaire.
First patrol On 18 June,
U-99 departed Kiel for operations in the
North Sea west of Norway. An
Arado Ar-196 seaplane from the German battlecruiser attacked her, having mistaken her for a British submarine. Two days later,
U-99 was attacked by two aircraft; minor damage was inflicted. She returned to Kiel on 25 June.
Second patrol U-99 departed
Wilhelmshaven on 27 June to patrol southwest of Ireland. On 29 June, she was attacked by British aircraft. A crash dive was carried out with the result that the boat hit the seabed, causing some damage which was able to be repaired. An attack on the was called off on 7 July, when the
defensively equipped merchant ship returned fire. On 8 July, over 100
depth charges were dropped by the escorts of Convoy HX 53, but
U-99 escaped undamaged. The patrol ended on 21 July. † Convoy HX 52, ‡ Convoy HX 53, * sunk by German bombing, 15 July 1940
Third patrol On 25 July,
U-99 departed Lorient for the North Atlantic. Four ships were sunk and three others damaged. On the 31st, the escorts of Convoy OB 191 dropped 20 depth charges on the boat without effect. Later that evening, a
flying boat also attacked her, again without causing any damage. The patrol ended on 5 August. † Convoy OB 191, ‡ Damaged
Fourth patrol U-99 departed Lorient on 4 September for the North Atlantic; seven ships were sunk. The patrol ended on the 25th.
U-99 was slightly damaged in an
air raid on Lorient on 27 September. † Convoy SC 3, ‡ Convoy HX 71, *
Convoy HX 72 Fifth patrol On 13 October,
U-99 departed Lorient to patrol the North West Approaches. Six ships from
Convoy SC 7 were sunk and another was damaged. The patrol ended on 22 October.
Sixth patrol U-99 departed Lorient for the North West Approaches on 30 October 1940; four ships were sunk. The patrol ended on 8 November. † Convoy HX 83
Seventh patrol On 27 November,
U-99 departed Lorient for the North Atlantic. Four ships were sunk. The patrol ended on 12 December. †
Convoy HX 90, ‡ Convoy OB 252
Eighth patrol U-99 departed Lorient on 22 February 1941 to patrol in the North Atlantic; eight ships were sunk.
U-99 was attacked herself, with severe damage inflicted. Kretschmer surrendered and scuttled the submarine with the loss of three lives. †
Convoy OB 293, ‡
Convoy HX 112 ==Fate==