The submarines of the Type IX class were built by modifying and improving the project of the Type 81 class built during the First World War. The Type IX B subclass was in turn obtained by increasing the operational range of the Type IX A subclass. Along with the submarines of the Type VII and Type XIV classes, the submarines of the Type IX class constituted the backbone of the German submarine offensive during the Second World War. Germany was at the head of the naval forces of the different countries with its submarine program, after Great Britain had unilaterally legalized the possession of submarines by Germany within the Anglo-German Naval Treaty of 1935. Germany had claimed equality with the countries of the British Commonwealth, stating that unless the "situation would require it", Germany would not be allowed to build more than a 45 percent of the total tonnage of the submarines built by the aforementioned countries. Sir Samuel Hoare, who back then was the Secretary of the Foreign Office, stated that the Naval High Staff did not object anything, as they considered that in that time Great Britain had totally dominated the submarine problem. This complacency towards the German plans nearly led Great Britain to its defeat.

The submarines of the Type I A, Type II A, Type II B, Type II C and Type II D subclasses were simple school boats in comparison with those of the Type VII A and subsequent classes, the first of which was laid down in 1936, according to a project which had been started some years before. The submarines of the latest subclasses were magnificent units which, with a speed of 17 knots in surface and a great maneuverability due to their double rudder, were ideal for surface attacks and, with an immersion time of 18 hours when navigating at 4 knots, equivalent to any contemporary submarine. The weakness of these submarines lied mainly in the placement of the compressed air bottles in places where they could be easily broken during an attack. It was noteworthy as well the discomfort in the centerline corridor and in every habitable place of these submarines.

The attack submarines of the Type VII A, Type VII B and Type VII C subclasses were very similar to each other, with the latter type having a more reinforced pressure hull. The Type VII D was elongated 10 meters to allocate five mine containers with three mines each, whereas the Type VII F was of similar length but transported twenty-five spare torpedoes to resupply other submarines in the high seas. The Type IX A, Type IX B, Type IX C and Type IX C/40 were subclasses of longer hull than those of the Type VII. The submarines of the Type IX D subclass were 11 meters longer than those of the former subclasses and had two slow-march Diesel engines instead of the six Diesel engines installed in those. This arrangement granted to these submarines an operational range in surface of 20580 nautical miles at 12 knots. Some units of the Type IX D subclass were used to transport necessary goods from Japan, but they were mostly used as submarine tankers and many of them were lost in this role. Ten units of the Type XIV class were later built as submarine tankers, and often called "milky cows". None of them survived the war. Each of them transported 437 tonnes of fuel and four spare torpedoes. They markedly contributed to the success of the operations carried out in the awfully unsuitable eastern coast of United States in the early 1942. The 22nd and 23rd April 1942, fourteen submarines could resupply in an area of 800 kilometers away from the Bermudas, so eighteen German submarines could operate between Cape Sable and Key West during May. However, these "milky cows" were particularly vulnerable and, despite their greater survivability during immersion, nine units were victims of air attacks and only one was destroyed by surface ships.