The Type XXI, was designed from the beginning as a true submarine, whose natural habitat was in the depths.
Almost everything about the submarine was new and out of all proportions, achieved unconventional underwater performance
far beyond the capabilities of any submarine then either in service or under development. She was designed to have a faster speed
submerged than when running on the surface. Equipped with air-conditioning, she was designed to spend most of her time underwater
and could remain submerged for up to 11 days at a time, briefly surfacing for only3 to 5 hours to recharge her batteries.
For this, the Type XXI had a full streamlined outer hull and complete absence of clutter on the deck.
The forward hydroplane retracted when not in use, there were no deck guns, the twin 20mm AA flak
were mounted in streamlined housings, and all extending devices such as snorkel,
antenna, and DF loop retracted into the superstructure when not in use. Instead of the traditional open conning tower,
there were three small openings at the top of the bridge, one for the watch officer and the other two for lookouts.

Internally, the cross section of the pressure hull was a figure of eight, with the upper section being of greater diameter than the lower.
The batteries were housed in the lower section. She had three times the battery capacity and with her new creep motor,
the Type XXI was very silent when running underwater. By comparison, the Type XXI at 15 knots emitted the same noise as
a US Navy Balao class boat at 8 knots. Her pressure hull was fabricated from 1 inch thick steel aluminum alloy, which allowed
a maximum crush depth of 280 meters (919 feet); the deepest of any military submarine at that time. The streamlined hull also
offered a much smaller sonar signature and with her silent running capability, and high underwater speed,
she was a much more difficult boat for enemy ASW vessels to find or detect. Equipped with a sophisticated echo chamber,
which could identify, track and target multiple vessels, the Type XXI could fire blind from up a depth of 160 feet.
Her firepower was also increased significantly.

With a new rapid reloading hydraulic system, the Type XXI could launch three six torpedo salvos
or eighteen torpedoes in just under 20 minutes; whereas it took over ten minutes to reload just one tube on the Type VIIC.
This meant that the Type XXI could attack more vessels in a single engagement.
The increased space also allowed more torpedoes to be carried – 23 instead of 14 on the Type VIIC.