DD tanks were a type of amphibious

Amphibious vehicle

An amphibious vehicle , is a vehicle or craft, that is a means of transport, viable on land as well as on water – just like an amphibian....

 swimming tank

Tank

A tank is a tracked, armoured fighting vehicle designed for front-line combat which combines operational mobility, tactical offensive, and defensive capabilities...

 developed by the British during the Second World War. The phrase is mostly used for the Duplex Drive variant of the M4 Sherman

M4 Sherman

The M4 Sherman, formally Medium Tank, M4, was the primary tank used by the United States during World War II. Thousands were also distributed to the Allies, including the British Commonwealth and Soviet armies, via lend-lease...

 medium tank

Tank

A tank is a tracked, armoured fighting vehicle designed for front-line combat which combines operational mobility, tactical offensive, and defensive capabilities...

, that was used by the Western Allies during and after the Normandy Landings in June 1944.

DD tanks worked by erecting a 'flotation screen' around the tank, which enabled it to float, and had a propeller powered by the tank's engine to drive them in the water.

The DD tanks were one of the many specialised assault vehicles, collectively known as Hobart's Funnies

Hobart's Funnies

Hobart's Funnies were a number of unusually modified tanks operated during World War II by the United Kingdom's 79th Armoured Division or by specialists from the Royal Engineers. They were designed in light of problems that more standard tanks experienced during the Dieppe Raid, so that the new...

, devised to support the planned invasion of Europe.
Amphibious tanks were devised during the First World War, a floating version of the British Mark IX tank

Mark IX tank

The Mark IX tank was a British armoured fighting vehicle from the First World War, the world's first specialised Armoured Personnel Carrier .-Development:...

 was being tested in November 1918, just as the war ended.