Three boats protected the transfer of the British Expeditionary Force to France in 1914, but were soon relegated to local defence and training duties.

The arrival of more modern submarines to blockade the Dardanelles made the B-class boats redundant and they were mostly withdrawn to Malta in 1915.

Their additional size increased their buoyancy and made them far less liable to unexpectedly plunge beneath the surface in bad weather.

The B-class submarines lacked any internal bulkheads which exposed the crew to the petrol engine's exhaust fumes.

Submerged the submarine used an electric motor powered by 159 battery cells at a working voltage of only 100 volts.

This originally lasted only three hours and 45 minutes at full speed, but this time was continually increased over the career of the boats as more powerful batteries were fitted.

They carried a maximum of 15.5 long tons (16 t) of petrol that provided a range of 740 nautical miles (1,370 km; 850 mi) at a speed of 8.7 knots (16.1 km/h; 10.0 mph).

Space was provided for a pair of reloads, but the addition of extra equipment over the years meant that they could only be carried if an equivalent weight of fuel was discarded.

[9] By the beginning of World War I, B3, B4, and B5 were at Dover and were deployed on a line between Calais and the Goodwin Sands to protect the passage of the British Expeditionary Force to France.

B3 and B4 were transferred to Ardrossan by the end of 1915 to relieve two even more obsolescent A-class boats on local defence duties in the Firth of Clyde.

Special guards were fitted over the forward diving planes to make sure that any mooring wires of mines would not get entangled and dragged down to the boat.

Shortly after departure one of the guards broke and began banging against the hull, making a lot of noise and vibration.

They succeeded before dawn revealed their presence to the Turks and proceeded up the Dardanelles at a depth of 80 feet (24.4 m) to avoid the Turkish minefields.

During one incident on 16 August 1915 a party of Arabs and officers in European uniforms were spotted displaying a flag of truce.

[15] In September 1915, after the Italian declaration of war on the Central Powers, the transfer of the B-class boats to the Adriatic was suggested by the Admiralty.

Nothing of note was directly accomplished by the submarines, but they fulfilled the ultimate goal of keeping the Austro-Hungarians from attacking the Italian coast.

B9 was spotted by an Austrian seaplane on 29 March and eventually forced to dive after both sides failed to damage each other with machine gun fire.

B7 had a narrow escape off the Austrian naval base at Pola three months later when she was damaged by another seaplane that jammed her diving planes in the "hard to rise" position which meant she could not submerge until they were repaired.

On one patrol the gear teeth of B11's steering mechanism were stripped which made the rudder loose and the boat unsteerable.

Slowly the boat began to drift towards the enemy coast, but a northwesterly breeze sprang up before dawn and the seas moderated which allowed her to make about 3 knots (5.6 km/h; 3.5 mph) by going astern.

[18] The B-class boats had reached the limits of their usefulness and the survivors were withdrawn to Malta on 30 October where they were paid off pending a decision on their fate.

They were assigned to patrol the Otranto Barrage, but their age and small size worked against them and they were soon paid off at Malta where they were sold after the war.