The B-class submarines were initially assigned to the Third Division of the Home Fleet, based at Portsmouth and Devonport, and were tasked with coastal-defence duties and defending the Straits of Dover in wartime.

After the Kingdom of Italy joined the Allies in May 1915, the B-class submarines in the Mediterranean were transferred to Venice to reinforce Italian forces in the northern Adriatic.

[6] B8, B7 and B9 were the first to arrive in Venice on 11 October, although B8 was immediately sent to the dockyard to repair damage suffered when she collided with the Italian tugboat Luni.

B8 was the first British submarine in the Adriatic to sight an enemy warship when she spotted an Austro-Hungarian destroyer off the Istrian coast on 8 November, although she was unable to attack it.

The five British submarines made a total of 13 patrols off the Austro-Hungarian coast before the end of 1915, hampered by bad weather and drifting mines, followed by 13 more in the first two months of 1916.

[7] B8 reported spotting a periscope and being missed by a torpedo while beginning a patrol on 28 February; postwar research has revealed that no enemy submarines were in the area where in the incident occurred.

Redesignated as S8 in August 1917, the boat was assigned to patrol the Otranto Barrage that was intended to prevent the Austro-Hungarian Navy from breaking out of the Adriatic, although she proved to be very unreliable in service.