The German U-boat onslaught against British merchant ships during the autumn of 1940 was highly successful because the attacks were made on the surface at night and from such close range that a single torpedo would sink a ship. Soon, though, Allied technology was able to detect U-boats at night, and new convoy techniques, combined with powerfully-armed, fast modern aircraft searching the seas, meant that by 1941 it was clear that Germany was losing the war at sea. Something had to be done. The new generation of attack U-boats that had been introduced since Hitler came to power needed urgent improvement. This is the story of the Types II, VII and IX that had already become the 'workhorse' of the Kriegsmarine's submarine fleet and continued to put out to sea to attack Allied shipping right up to the end of the war.