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.

[3] For surface running, the boats were powered by two 775-brake-horsepower (578 kW) diesel engines, each driving one propeller shaft.

[2] Ordered on 23 December 1932, Sealion was laid down on 16 May 1933 in Cammell Laird's shipyard in Birkenhead and was launched on 16 March 1934.

Her first success was the German merchant August Leonhardt, sunk in April 1940 off the Danish island of Anholt.

She was one of a number of submarines ordered to track the German battleship Bismarck before her eventual sinking.