[Rainer]

Tony, I wrote that the strategic bombing was the main reason for the delay to get the type XXI into service - not for the whole project.

The developement of the Walter propulsion had no high priority in the Kriegsmarine and it was in November 1942 when they decided to design the type XVIII to test if the propulsion could be used operational. Its double-hull design inspired two engineers (the same guys that designed the type VII) to replace the fuel tanks for the Walter propulsion with additional batteries, an idea which eventually led to the type XXI. But this took place in January 1943 and not already 1941, the design was finished in June 1943 and presented to Hitler on 8 July 1943. So the development of the XXI came just in time for the Kriegsmarine after their conventional submersibles were defeated in May 1943.

Speer's planning of the production reduced the time to built one type XXI from 18 to 6 months. Of course there were some serious troubles with the sub-contractors initially and the timely delivery of the components to the shipyards where they were assembled until these problems were largely solved in mid 1944, but at that time the strategic bombing campaign began to effect the completion of the new U-boats badly and hampered the training of the crews. At least 60 type XXI under construction were destroyed by Allied bombing raids together with 14 of the 118 U-boats of that type that were commissioned. In fact, only two type XXI had reached operational status when the war ended.