Type XXI submarines are considered the first "true" submarines because they were capable of conducting a wartime mission almost completely submerged. All previously developed submarines were more or less surface craft with the ability to operate submerged for short periods of time, mainly for attacking or dropping off the enemy.
Also crucial was the increase in submerged speed from 7.6 knots (of the Type VII C) to 17.2 knots.
Today's submarines, like the last German submarines of World War II, are also true submarines. Some of these operate underwater for several months without having to come to the surface.
"Wilhelm Bauer" is the only Type XXI submarine who can still be visited.