The Type IX U-boat was designed by Nazi Germany's Kriegsmarine in 1935 and 1936 as a large ocean-going submarine for sustained operations far from the home support facilities. Type IX boats were briefly used for patrols off the eastern United States in an attempt to disrupt the stream of troops and supplies bound for Europe. It was derived from the Type IA, and appeared in various sub-types.

The Type I U-boat was the first post–World War I attempt to produce an oceangoing submarine for Nazi Germany's Kriegsmarine. The type was based on the Spanish Type E-1, designed by Ingenieurskantoor voor Scheepsbouw. Only two Type IAs were built. They were not a successful design : due to its single rudder they had a large turning circle and were not very manoeuvrable. The gravity center of the U-boat was too forward so on surface the type I had its propellers exposed when she was pitching. Whilst submerged there were problems with depth keeping and stability as air bubbles in fuel tanks wobbled back and forth. Diving was very slow : at full speed and with six tons of negative buoyancy it took forty seconds to reach ten metres depth. As a result, the type was discontinued and a new type IX class oceangoing U-boats was designed.