Following World War 1 (1914-1918), the Paris Peace Conference of 1919 developed the Treaty of Versailles to formally restrict the total tonnage of the German naval fleet and forbid construction of new submarines (as well as tanks and aircraft). The German Government eventually developed various avenues in which to skirt the treaty limitations and this led to the setting up of local offices within the borders of several neighboring - yet neutral - countries; this proved the case when offices were opened in Holland and Sweden to further develop submarine and torpedo technologies. Therefore, even before the official start of World War 2, Germany had already enacted submarine (U-boat) building programs and proceeded to train new submariners in the dangerous craft. When these developments came to light, the German government responded to the world by labeling these projects as mere "experimental" endeavors (or simply outright attempting to conceal their war planning efforts through various guises). As time went on, the general lack of enforcement only served to encourage the German war machine to continue its building of submarines, warships, tanks and aircraft. Eventually the Anglo-German Naval Agreement of 1935 was brought along to simply accept the secret naval build-up and only limited the German submarine total to that of Britain's at the time.
One of the early U-boat programs delivered the "Type IA" submarine, the design based on proven qualities of a Finnish and Spanish design prior. The Type IA was considered an "ocean-going" submarine which did not limit its deep water operation (as opposed to "coastal" submarines). She displaced at 983 tons submerged and, by German Navy standards, was categorized as a "large" submarine despite her modest weight when compared to her contemporaries elsewhere. Only two Type IAs were eventually built for the class (U-25 and U-26) and construction was handled through A.G. Weser in Bremen, Germany. U-25 was officially commissioned on April 6th, 1936 and the class proved serviceable as attack submarines, generally resembling the basic U-boat designs of World War 1 with their flat decks and slim profiles. The bow sported a large serrated cable cutter for when navigating obstacles while the conning tower was fitted amidships. A single deck gun was mounted ahead of the conning tower for short-ranged surface attacks and a small caliber cannon was installed to defend against low-lying aircraft. It was only in the vessels' internal design that the class truly showcased some notable advances - a completely welded pressure hull and the selection of 21-inch (533mm) torpedos - the latter the first instance of the 21-inch torpedo international standard being adopted by the German Navy.
The torpedo served as the boat's primary armament, proving sound for engaging surface ships of many types. These early-war German World War 2 torpedoes were basic "straight runners" while, later in the war, homing- and pattern-running torpedoes were developed for improved accuracy. These were further fitted with two types of trigger; an impact type which detonates upon contact and a magnetic type which exploded within a few meters of the target - having "sensed" a change in its magnetic field. The submarine could also be laden with naval mines for specialized "denial" missions. The 105mm deck gun came into play for engaging low threat surface or land-based targets.
The submarine measured at 72.4 meters long, 6.2 meters wide and 4.3 meters high and could make upwards of 18 knots on the surface in ideal conditions (much less when submerged). She was outfitted with 4x21-inch torpedo tubes at the bow and 2x21-inch torpedo tubes at the stern - allowing the crew to engage targets both fore and aft, standard practice for World War 2 submarines. The boat carried 14 x torpedoes and 28 x mines along with its 1 x 105mm (10.5cm) SKC/36 series deck cannon. The cannon resided on the deck just forward of the conning tower. Anti-aircraft defense came in the form of a 1 x 2cm/30-37 (20mm) cannon mounted on the conning tower proper.
The Type IA series managed a fuel supply for approximately 8,100 miles of surface travel when running at 10 knots (range was drastically decreased when the vessel was submerged as more power was required). For defense against dedicated submarine hunters such as destroyers, the Type IA could dive to depths of 330 feet. The design furthermore supported a full complement of 43 crew including four officers. Propulsion stemmed from 2 x MAN 4-stroke diesel engines developing 3,080 horsepower while driving two screws. Undersea operation was made possible by 2 x BBC electric motors rated at 1,000 horsepower. Due to the reliance of batteries to power various systems (including oxygen) when submerged, the boat class was required to surface at regular intervals to reclaim oxygen stores and recharge battery energy - this always proved the submarine's most vulnerable time and often times done in the night time hours. With its experimental label, the Type IA design eventually suffered through mechanical deficiencies over the life of her service, proving generally unreliable.
Both U-25 and U-26 entered service in 1936 and were initially utilized as crew training vessels. However, the governing Nazi Party was not shy about showcasing the new submarine in news reports to help drive home the image of German superiority in all things. When war finally broke out in September of 1939 between Germany and Poland (the Battle of Westerplatte), both of the Type1A boats were put into operational service despite their "experimental" existence. The vessels were put into formal combat roles simply because the German Navy lacked many serviceable attack submarines at the outbreak of war.
On October 31st, 1939, U-25 sunk its first target - the Baoule of France with its 5,874 ton cargo. In January of 1940, U-25 then made its most successful sortie in Atlantic waters when she downed four enemy merchant ships - the Enid and Songa of Norway, the Polzella of Britain and the Pajala of Sweden for a total tonnage loss of 15,353 tons. U-25 then followed up with the Armanistan of Britain and the Chastine Maersk of Denmark in February. Her final "kill" was the HMS Scotstoun of Britain on June 13th, 1940. She managed to damage the French Brumaire on June 19th, 1940 before the end.
When called to participate in the German Invasion of Norway in August of 1940, U-25 was on a mine-laying mission when she herself fell victim to a naval mine. The damage proved severe enough that the entire vessel - 49 crew and all - was lost to the sea, thus ending the short tenure of the U-25. During her wartime service, she managed to sink a total of eight enemy ships while damaging another (the aforementioned Brumaire). The U-26 managed a slightly better operational career, sinking some 11 enemy surface ships before herself being scuttled on July 1st, 1940. Despite these successes, the Type IA design became largely remembered for their poor ocean-going qualities, general unreliability and inadequate handling at sea - all detrimental qualities to an attack submarine but common to experimental types. Regardless, experience in the design and development of the Type IA class served the Germans quite well for the upcoming Type VII and Type IX boat classes.