As there was no time for a completely new design, the Mark IX was based on the Mark V, with the hull lengthened to . The 150 hp Ricardo engine was moved to the front, the gearbox to the back and the suspension girders left out entirely. This created an inner space long and wide, enough room for thirty (officially even fifty) soldiers or ten tons of cargo. To ensure sufficient stiffness for the chassis, the floor was reinforced by heavy transverse girders. The infantry inside had to contend with the control rods for the gears running along the roof and the drive shaft through the middle. Unfortunately for the infantrymen, no seats were built in for them in the tank.