The Churchill AVRE (Assault Vehicle, Royal Engineers) was developed after the Dieppe raid in an attempt to make combat engineers less vulnerable while they were attempting to destroy enemy defences.
The AVRE was developed from a suggestion made by Lieutenant J. J. Denovan of the Royal Canadian Engineers, but attached to the Special Devices Branch of the Department of Tank Design. His idea was for a tank with as much of the standard internal equipment as possible removed and replaced with storage space for the sapper's equipment, tools and explosives. The Churchill was chosen because of its combination of a large interior, thick armour and side access door, and a prototype was developed for the Department of Tank Design by the 1st Canadian M E Company.
A demonstration on Hankley Common on 25 February 1943 showed what the engineers had in mind. A Churchill tank with the internal ammunition storage removed and a new side door that unfolded to become an armoured screen was driven up to a concrete wall. The sappers emerged from the tank, placed and lit General Wade explosive charges on the wall, and then retreated in the tank. The resulting hole was large enough to drive a tank through.
The 290mm muzzle loading mortar was developed separately, by Colonel Blacker, the designer of the Blacker Bombard, a spigot mortar built for the Home Guard. He was asked to design a version of the mortar that could be mounted on a tank, and produced a mortar that could fire a 40lb high explosive shell known as the Flying Dustbin. A massive spring soaked up the 20 tons of recoil and used the energy to recock the mortar. At the Hankley Common demonstration this mortar was mounted to a Churchill tanks. After using shells fused for air burst to clear a 28ft wide gap through a minefield, the mortar then fired twelve shells directly at a 6ft thick concrete wall, creating a gap wide enough for a tank.
The two designed were merged to create the AVRE. Around 700 were produced by converting Churchill Mk IIIs and IVs, of which 180 had been completed by the D-Day landing, where they were used by the 1st Assault Brigade of the 79th Armoured Division. The AVRE was given standard attachment points that could be used to carry a wide range of specialised equipment, including fascine carriers that could drop their brushwood bundles into ditches or at the base of barriers, a variety of mine sweeping devices, a Small Box Girder bridge, 'bobbin' carpet laying tanks and the 'Goat' explosive device.
The AVRE played an important part in the success of the British and Canadian landings on D-Day, where their spigot mortar was especially valuable, destroying a number of German strong points, the most famous being the Sanatorium at Le Hamel. They continued to operate successfully during the campaign in north-western Europe, and later versions of the AVRE tank remained in use long after the Churchill had been retired.