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Only two boats of this class will be built for the Kriegsmarine, the U-25 and U-26; the U-26 went into commission IRL on 11 May 1936. Her first captain was Kapitänleutnant Werner Hartmann.

Length 237', beam 20', displacement (surfaced) 862 tons.

Surface speed 18.6 kts, submerged 8.3 kts; range 7900 nautical miles at 10 kts, submerged 78 nautical miles at 4 kts; max designed depth 650'. Crash dive time, 30 seconds.

Armament: 4 bow and 2 stern torpedo tubes; 14 torpedoes carried; also 38 TMA mines, and a 105mm deck gun with 150 rds of ammunition.

Crew: 45.

These vessels have been "found to have unsatisfactory seakeeping qualities: unstable both above and below the surface, slow to dive and difficult to manoeuvre underwater. They were also found to be extremely unreliable mechanically, several wartime patrols cancelled due to myriad small defects. The boats were so troublesome that Dönitz wrote in his War Diary on 20 December 1939, 'U25, the boat, that has only managed a short patrol since the war's beginning and spent the rest of its time in the shipyard, must again break off its trip and return to the yard.'" 

• The G7a torpedoes has a range of 6 km at 44 kts (or 8 km at 40 kts, or 14 km at 30 kts); as a "compressed air" torpedo it leaves a visible trail of bubbles on the way to the target. Warhead, 616 pounds; cost 20,000 marks (about $7500 in 1934).

• The TMA is a moored mine, which means that it is secured by a heavy anchor and a cable attached to the mine. The mine floats at the surface, while the anchor secures its position in the sea (maximum 885'). Two can be loaded in each torpedo tube. Warhead, 473 pounds.