Because of the restrictions placed upon Germany's fighting forces at the close of World War 1 (1914-1918), the rebuilding military was forced to get creative during its rearmament period in the 1930s. This sometimes meant conducting research, development and production on foreign soil and eventually bringing the completed product back home. This proved the case with the Type II U-boat for it was born in Finland and developed from the private venture CV-707 "Vesikko" submarine of 1933. The German version, differing mainly by way of a reduced draught, reduced operational range and slower submerged speeds, arrived in 1934 and saw commissioning begin in 1935.
The series was more or less inspired by the British success with similar coastal types in World War 1. As such the German design was engineered with excellent agility and a compact footprint for their primary role would be to operate close-to-shore ("littoral"), patrolling for enemy value targets either on the surface or under it. In June of 1935, the rearming Germans claimed a political victory when it forced the British to sign a new Anglo-German agreement which allowed for greater expansion of the German Navy service - now within a new 45% limitation against the current size of the Royal Navy fleet.
Fifty of these small boats were ultimately built and arrived in four distinct standards - Type IIA (6 units), Type IIB (20 units), Type IIC (8 units) and Type IID (16 units). The series was constructed from 1934 into 1940 through a group effort, managed by the shipyards of Deutsche Werke, Germaniawerft, Flender Werke and Galati. Commissioning of the class spanned from 1935 until 1945. The initial production form was Type IIA but an inherent endurance limitation gave rise to the Type IIB which improved upon this quality by adding fuel stores. The Type IIC then arrived and was essentially the Type IIB with more powerful machinery. The Type IID differed as it added "saddle tanks", these to either side of the hull for improved ballast functionality.
Externally, the boats were conventional in arrangement with the fin set over midships, the bow featuring the usual elevated form (with a flat-top, walking deck) and the rear containing the rudder facilities and twin propeller units. Internally, the propulsion scheme included 2 x MWM RS127S 6-cylinder marine diesels outputting 700 horsepower for surface traveling and 2 x SSW PGVV322/36 double-acting electric motors of 410 horsepower output for submerged work. The boat could hit surfaced speeds of 13 knots and submerged speeds of 7.5 knots out to a range of 4,040 miles (surfaced, otherwise 65 miles submerged). The submarine had a capability to "crash dive" in just 25 seconds - a useful trait to have when attempting to disappear from enemy view. The crew numbered twenty-five men.
Because of the boats small size, armament was limited to 3 x 533mm (21") bow-facing torpedo tubes with six reloads available. Naval mines could also be carried as well but this in place of the torpedo load. For surface work, the boat was originally equipped with 1 x 20mm Anti-Aircraft (AA) gun and this later gave way to as many as four such weapons as the war drew on.
Such was the speed at which the German Navy grew that the Type II was put into series production almost as soon as it was available. However, the coastal-minded design of the Type II ended up restricting its tactical value once the war had moved from sea to ocean. This left the Type IIs with little offensive role to play in the evolving war and many of the lot were held back for training duties or to be used in testing out new submarine-oriented components being trialed by the Germans.
Type IIA boats ran from U-1 to U-6. Type IIB boats followed form U-7 to U-24 and U-120 and U-121. The Type IIC boats were U-56 through U-63. Type IID boats were U-137 to U-152.