On 4 December 1939, Salmon became the first boat to sink a U-boat during the Second World War when it torpedoed and sank the German U-36 in the North Sea south-west of Kristiansand, Norway.
[1] The second batch of S-class submarines were designed as slightly improved and enlarged versions of the earlier boats of the class and were intended to operate in the North and Baltic Seas.
[4] For surface running, the boats were powered by two 775-brake-horsepower (578 kW) diesel engines, each driving one propeller shaft.
[3] Ordered on 20 January 1933, Salmon was laid down on 15 June 1933 in Cammell Laird's shipyard in Birkenhead and was launched on 30 April 1934.
[8] Bickford's decision not to fire on Bremen likely delayed the start of unrestricted submarine warfare in the war.