The submarines were ordered in response to the rising German threat to the south in the interwar period.

Sjölejonet was armed with six 533 mm (21 in) torpedo tubes, three of which were situated in the bow, one in the stern and the remaining two in a single traversing deck mounting.

[2] In the 1950s, Sjölejonet underwent a major refit where the guns and deck torpedo tubes were removed and the conning tower was streamlined.

[2] The submarine's displacements remained the same, now 660 tonnes (650 long tons) surfaced and the diesels capable of 2,200 kW (3,000 bhp) and electric motors 1,500 kW (2,000 hp), but only a maximum speed of 15 knots (28 km/h; 17 mph) surfaced.

[3][a] Constructed by Kockums MV A/Bat their shipyard in Malmö, Sweden, Sjölejonet's keel was laid down in 1935.