From the tests of the A6, three Mark IIIs were built and put into service with the British Army but due to the high cost no more were purchased.

In 1926, the British War Office wanted to replace their existing Mark II tanks with a new design.

Other specifications included that it could transported by rail, a sufficient supply of lubrication oil to match the range of the tank (dictated by the fuel carried), a wireless set, a gun capable of defeating enemy armour at a range of at least 1,000 yd (910 m), fuel tanks external to the main compartments and bottom armour sufficient to withstand heavy machine-gun fire, when exposed climbing a crest.

There would be a central two-man turret with a 3-pounder (1.9 in (47 mm)) gun and a coaxial machine-gun; it was intended to house the commander and a 'special observer', with a cupola for each.

A6E1, A6E2 and A6E3 were fitted with an Armstrong Siddeley air-cooled V8 180 hp engine giving a maximum speed of 26 mph (42 km/h).

It was proposed to combine two Rolls-Royce Phantom engines with the Wilson transmission system on the A6E1 but this was rejected on grounds of expense.

The AA-turret was removed from A6E1 but the suspension and the gunnery arrangements were distinctly inferior to those of the Mark II.

The secondary machine-gun turrets were moved more to the front to shift the centre of gravity of the vehicle forward to improve its stability and larger brakes were fitted.

The failings of the suspension continued; even though road speed increased to 30 mph (48 km/h), the bogies were often overloaded during cross-country rides.