The Sherman Duplex Drive tanks were part of a range of unconventional armoured vehicles developed by the Allies to breach defences, clear minefields and push inland on the day of the invasion.
Much to the annoyance of Major-General Percy Hobart, commander of 79th Armoured Division operating most of these vehicles, and a fierce exponent of armoured modernisation, British soldiers had christened them 'funnies'.
The DD tanks were the brainchild of a Hungarian, Nicholas Straussler. They were fitted with a canvas screen around the hull which displaced enough water to allow them to float, and were powered by tracks on land and a rear propeller at sea. On D-Day they enjoyed mixed fortunes - many were swamped by heavy seas after being launched off the beaches, and sank with their crews. But others made it ashore to direct high-velocity firepower at a crucial early stage of the landing.
The DD tanks were developed after the disastrous raid on Dieppe in 1942 when many of the tanks taken ashore were unable to leave the beaches because of defence obstacles. It was known that similar defences had been erected along the Normandy coast. So specialised armoured vehicles were developed to precede the infantry during the landings so that obstacles, pillboxes and mines could be cleared and exits from the beaches established.
The Americans decided against using 'funnies', apart from DD tanks and armoured bulldozers, and their infantry landed without adequate armoured support, suffering heavy casualties.
The principal workhorse on the British and Canadian beaches was the AVRE (Assault Vehicle Royal Engineers), a Churchill tank armed with a Petard which could throw a 40lb bomb - the 'Flying Dustbin' - to destroy concrete bunkers.
The AVREs had a range of specialised equipment to deal with obstacles, soft ground and sand dunes. These included assault bridges, bundles of palings or faggots to fill ditches, carpet layers for soft beaches, devices to explode or lift mines, demolition charges and ramps.
The most efficient vehicle for clearing anti-tank mines was the flail tank, known as the 'crab'. This was a Sherman fitted with a revolving drum at the front to which chains were attached to beat the ground in its path and detonate the mines. On 6 June, four squadrons of assault engineers and two regiments with flail tanks landed on the British and Canadian beaches. The 'funnies' overwhelmed many defences and astounded the Germans, who had nothing comparable.
The 'funnies' were regarded as so crucial to the North-west Europe campaign that much of their work was not publicised until much later - the existence of DD tanks was not officially admitted until after May 1945.