By coincidence the First World War-vintage H-class submarines used for training in anti-submarine warfare were reaching the end of their useful service.

The Rear-Admiral Submarines, Noel Laurence, wanted a class of small, inexpensive boats for training, armed with torpedoes for short-range patrols.

[3] The three Unity-class boats, HMS Undine, Unity and Ursula were ordered on 5 November 1936 from Vickers-Armstrongs, to be built at their Naval Construction Yard in Barrow-in-Furness.

The superstructure and conning tower were built with free-flooding holes and storage for cables, anchors and sundry items.

The hull was divided by five bulkheads with access from the conning tower; hatches in the torpedo-stowage compartment and in the engine room had drop-down canvas trunks for emergencies.

[10] The boats had a fuel capacity of 38 long tons (39 t) giving a range of 3,800 nmi (7,000 km; 4,400 mi) at 10 kn (19 km/h; 12 mph) on the surface and 120 nmi (220 km; 140 mi) at 2 kn (3.7 km/h; 2.3 mph) submerged; battery recharging required the submarine to surface; in 1944 dummy snorkels were fitted to some boats for anti-submarine warfare training[11] During construction the four internal bow torpedo tubes were supplemented by two external tubes in a bulged housing, four reloads being carried for the internal tubes.

The sudden loss of weight in the bows when the torpedoes were loosed in salvo made the boat porpoise and break the surface.

Numbering submarines had been the practice in the First World War but because of Churchill's views, the Admiralty decided that it was better to be right than consistent and naming was resumed.

[18] The group included submarines that became well-known; Urchin was transferred to the Polish Navy as ORP Sokół and sank 55,000 long tons (56,000 t) of Axis shipping.

Wanklyn was awarded the Victoria Cross for attacking a well-defended convoy and sinking the Italian liner Conte Rosso on 25 May 1941.

[25] The V-class boats were the final refinement of the U-class submarines, 34 were ordered and 21 were built by Vickers for the War Emergency Programmes of 1941 and 1942, the rest being cancelled.