They were designed to be propelled by diesel motors on the surface to avoid the problems with petrol engines experienced with the A class.

These boats were designed for foreign service with an endurance of 2,500 nautical miles (4,600 km; 2,900 mi) at 10 knots (19 km/h; 12 mph) on the surface and much improved living conditions for a larger crew.

The D-class submarines were considered to be so innovative that the prototype, D1, was built in utmost secrecy in a securely guarded building shed.

She was launched at Barrow with equal secrecy, with only departmental heads and a few officers from the cruiser HMS Mercury, that was currently in dock being present.

During World War I the boats patrolled the North Sea and the Heligoland Bight, and protected cross channel troopships.