[2] In September 1917 Armstrong, Whitworth & Co. in Newcastle-upon-Tyne began constructing two prototypes of a pure transport vehicle that would become the Mark IX, which could also serve as a supply tank.

The prototypes were approved the following year, at a time when it had become clear that a possible alternative, the stretched Mark V* tank, was unsuited for infantry transport.

Two hundred Mark IXs were ordered from the tractor manufacturer Marshall, Sons & Co. of Gainsborough, Lincolnshire but by the end of the Great War only three had been finished, out of a total ultimate production run of thirty-four.

Designed as an armoured personnel carrier, the type had elements of an infantry fighting vehicle, as along each side of the hull there were eight loopholes, through which the soldiers could fire their rifles.

The tank could also carry supplies in a tray on the roof behind the commander's armoured observation turret (being the highest point at 8.7 feet (2.64 m)), while towing up to three loaded sledges.

[3] The type – named The Pig after the low front track silhouette that gave a snout-like appearance[4] – was used as the basis of two conversions: one of the first three built was used as an armoured ambulance,[3] while another was rebuilt as an amphibious tank by the staff of the test base at Dollis Hill.

[5] Already a bulky vehicle — the probable reason the Mark IX was selected as the basis for an amphibious tank — its displacement was improved by fitting drums at the front and sides.