M4A1 DD is a United States Tank unit.
The ubiquitous M4 Sherman medium tank evolved from the M3 Lee medium tank pressed into service as a stop-gap model in 1941. The M4 entered service in late 1942, outperforming its older sibling in every aspect, the Sherman went on to become one of the most widely produced armored fighting vehicles of World War II. It was well armored, relatively fast, and cheap to produce, becoming the backbone of Allied armored divisions on the Western Front.
The M4A1 sub-designation indicates a Sherman model manufactured with a fully cast upper hull, a standard Continental R975 radial engine, and a 75mm medium-velocity general-purpose gun. The DD stands for Duplex Drive (or Donald Duck among troops), indicating a Sherman fitted with a flotation screen and a propeller, allowing it to cross water barriers.
DD tanks primarily served during D-Day operations, but a number of them were later used in Western Europe for river crossings.