The D3E1 "Wheel-cum-Track" Machine was an inter-war experimental tank design by the Vickers concern of Britain with the project goal of developing a new method to extend the life of track sections on combat vehicles. The design was requested from Vickers by the War Office in 1927 and involved an armored car-like body (conventional roadwheels and all) affixed to a track-and-wheel drive scheme. The idea behind the concept was to allow the vehicle to use its roadwheels during general road travel and its tracks for cross-country travel. The concept proved too complicated and was ultimately abandoned.
As this Vickers experimental utilized both types of running equipment, it was required to carry separate control and drive systems for each arrangement. When on its roadwheels (involving all-solid-tires), drive power was sent to the rear axle only and steering accomplished through the front axle. An Armstrong Siddeley gasoline engine was used to power the vehicle and road speeds could reach up to 30 miles per hour on its roadwheels and up to 13 miles per hour when relying on the track-and-wheel units. A pair of turrets were fitted onto the armored car body with the forward, primary, larger installation showcasing a pair of machine guns over midships and the secondary, smaller aft turret outfitted with a single machine gun.
However, beyond its inherent mechanical complexity, the track sections were deemed too short for the long wheelbase at play, making for a wide turning radius. Comfort for the crew was low as the solid tires gave little cushion. The D3E2 emerged as a second prototype delivered by Vickers but given an open-air armored hull.
Both of these examples were eventually dropped from serious consideration.