Why Type XXI is unlikely by 1942

Would a delayed war give Germany Type XXI submarines by 1942?

Probably not, because Type XXI development depended in part on research Germany acquired through later conquests.

Three extra peacetime years do not automatically produce the Type XXI. The historical development path drew on research acquired after Germany occupied the Netherlands. Without that capture and the pressures of an ongoing submarine war, the same boat cannot simply be moved into a 1942 fleet. Continued Type VII production and further Type IX development are treated as likelier outcomes.