The submarines of the Type IX were built by modifying and improving the project of the Type 81 built during the First World War. The Type IX B was in turn achieved by increasing the operational range of the type IX A. Along with the submarines of the types VII and XIV, they constituted the base 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 Britain had unilaterally legalized the possession of submarines by 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 so", Germany would not be allowed to build more than a 45 percent of the total of the ships built by those aforementioned countries. Sir Samuel Hoare, then Secretary of the Foreign Office, stated that the Naval High Staff did not object anything, considering that in that time Great Britain had totally dominated the submarine problem. This complacency towards the German plans nearly led Britain to the defeat.

The submarines of the types I A, II A, II B, II C and II D were simple school ships in comparison with the ones of the types VII A and subsequent, the first of which was laid down in 1936, according to a project that had been started some years before. The submarines of the last types resulted magnificent units which, with a speed of 17 knots in surface and great maneuverability thanks to their double rudder, resulted ideal for surface attacks, and equivalent to any contemporary submarine with an immersion time of 18 hours at 4 knots. 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 also notable the discomfort in the central corridor and in every habitable place of the submarine.

The types VII A, VII B, and VII C were attack submarines very similar to each other, 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, while the Type VII F was of similar length but it carried 25 spare torpedoes to resupply other ships in the high seas. The types IX A, IX B, IX C and IX C/40 were series of longer hull, but the submarines of the Type IX D were 11 meters longer and they had two slow-march Diesel engines instead of the six on the former types.

This disposition provided to these submarines an operational range in surface of 20580 nautical miles at 12 knots. Some units of the Type IX D were used to transport necessities from Japan, but they were mostly used as submarine tankers, and in this mission many of them were lost. They were later built ten units of the Type XIV as submarine tankers, often called "milky cows". None of them survived the war. Each of them transported 437 tonnes of fuel and four torpedo reloads. 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, 14 submarines could resupply in an area of 800 kilometers from the Bermudas, so 18 German ships could operate between Cape Sable and Key West during May. However, these "milky cows" were particularly vulnerable and despite of a better survivability during immersion, nine units were victim of air attacks and only one was destroyed by surface ships.