The Mark IX tank was classified as an armored personnel carrier - effectively the world's first of its kind anywhere in operation service. The system was designed to a purpose-specific requirement needing an armored vehicle capable of transporting combat-ready troops or supplies to the front under protection. A wrinkle in the specifications, however, complicated early design as the Mark IX would also have to be made "tank-ready" meaning the design should serve as an APC but allow for conversion to a tank where the existing APC hull could be fitted with side sponsons sporting offensive-minded armament. This additional requirement stemmed from the fear that another armor project - the Mk VIII, a purpose-built armored fighting vehicle (AFV) - might not live up to expectations. As such, the back-up plan of using a multi-purpose hull was deemed necessary in the Mark IX design.
Based on the previous (and existing) Mark V tank design, the Mark IX was rushed onto the drawing boards. The engine of the Mark V was flipped with the gearbox, sending the engine forward and the gearbox to the rear. Crew accommodations consisted of four standard operating personnel with a utilitarian cabin space for up to 30 soldiers. The crew was made up of a commander, driver, mechanic and the machine gunner. The cabin space offered up no comforts whatsoever, not so much as seating for personnel or a compartment separating their area from the noise, grease and smoke from the operating engine - and facilitated the dual-use ability of the vehicle to carry people or supplies aboard without the need for conversion. Self-defense was basic with two 7.7mm Hotchkiss type anti-infantry machines guns with one held in a forward position and the other at rear, operated through nothing more than hatches. The passengers could jump into the fray as well for each side contained eight rounded hatches for which the business end of a rifle could be passed through and fired on the enemy. Power was derived from a single Ricardo 6-cylinder gasoline engine of 150bhp. Combined with the armor, this gave the vehicle a top speed of nearly 3.5 miles per hour in optimal road and weather conditions.
Externally, the Mark IX tank fit the role. It was featureless with the exception of the four noticeable rounded crew cabin doors (two to a side). The hull and attached superstructure was nestled in-between the track assemblies with the tracks taking on the trapezoid shape consistent with tank designs of the Great War. The large rearward area of the upper hull could be used to carry equipment and supplies in addition to what was held in the interior hold and total supply hauling capabilities topped 10 tons. Armor thickness reached 10 millimeters and, surprisingly, no type of suspension was provided.
Construction was under way in September of 1917 though only some 30 or so systems were actually produced before the end of the war in November of 1918. They were still considered of sound design and used for a time thereafter, though eventually done in by the general progression of armored vehicle designs and more purpose-built systems developed after the war. Regardless, the Mark IX proved to be a proper conversion of an existing design with haste necessitated by the calls of war. Its troop-carrying and supply-moving capabilities surely made up for any design-related drawbacks and rudimentary crew/passenger comforts.
The Mark IX was affectionately known as "The Pig" due to its distinct front-end shape. Construction was handled by Armstrong, Whitworth & Company of Newcastle-upon-Tyne.