The Mark VIII kept many of the general features of the Mark I-V series: it had their typical high track run and no revolving turret but two sponsons, one on each side of the tank, armed with a 6-pounder (57 mm) gun.
But it also resembled the Mark VI-project in that it had more rounded and wider tracks and a large superstructure on top directly beneath the front of which the driver was seated.
An innovative feature was the departure from the concept of the box tank with its single space into which all accessories were crammed.
The Mark VIII was compartimentalised with a separate engine room at the back.
This vastly improved fighting conditions as a bulkhead protected the crew against the deafening engine noise, noxious fumes and heat.

There were no machine guns in the sponsons, only the 6-pounders each manned by a gunner and loader.
The side machine guns were to the rear of the sponsons mounted in the hull doors.
Major Alden had designed the sponsons to be retractable (they could be swung in at the rear by the crew, being pivoted at the front), to reduce the width of the vehicle if enemy obstacles were encountered.
Five more machine guns were in the superstructure: two at the front—left and right next to the driver—and one on each of the other sides.
As there was no machine gun position covering the back of the tank there was a dead angle vulnerable to infantry attack.
To solve this problem a triangular steel deflector plate was attached.
The rear superstructure machine gunner could use it to deflect his fire down into that area behind the tank.
The tank carried 208 shells and 13,848 machine gun rounds, mostly in a large ammunition locker in the centre which formed a platform on which the commander stood behind the driver observing the battlefield through a cupola with four vision slits.
Later the side superstructure guns were removed on US tanks.

The twelfth crew member was the mechanic, seated next to the 300 hp Liberty V-12 (or in British tanks Ricardo V-12) petrol engine) cooled by a large horizontal radiator.
Three armoured fuel tanks at the rear held 200 Imperial gallons (240 US gallons, or 909 litres) of fuel giving a range of 89 km.
The transmission used a planetary gearbox giving two speeds in either forward or reverse.
Top speed was 5.25 mph (8 km/h).

To improve its trench crossing ability to 4.88 m the vehicle had a very elongated shape.
The track length was 34 ft 2 in (10.42 m) but even though the hull width was an impressive nominal 3.76 m, the actual length-width ratio of the tracks was very poor as that width included the sponsons.
Combined with wide tracks it proved difficult to turn the tank.
During testing many tracks twisted and broke in a turn and it was decided to use longer, stronger 13.25 inch (337 mm) links made of hardened cast armour plate, stiffened by webs formed by recesses in the track plate.
Another effect of the narrow hull was that the fighting compartment was also very narrow.
This was made worse by the fact that now the gap between the double track frames at each side was very wide; earlier types had only the tracks themselves widened.
Nevertheless, the tank was supposed to accommodate another twenty infantry men in full gear if necessary.
In absolute terms the vehicle was very large: at 10 ft 3 in (3.13 m) tall the Mark VIII was the second largest operational tank in history, after the Char 2C. However its weight was only 38.3 long tons (38.9 t) fitted for battle as the armour plate was thin with a thickness of 16 mm on the front and sides—a slight improvement over the Mark V but very thin by later standards.
The roof and bottom of the hull were protected by only 6 mm thick armour plate, leaving the tank very vulnerable to mortar shells and landmines.
The Mark VIII kept many of the general features of the Mark I-V series: it had their typical high track run and no revolving turret but two sponsons, one on each side of the tank, armed with a 6-pounder (57 mm) gun. But it also resembled the Mark VI-project in that it had more rounded and wider tracks and a large superstructure on top directly beneath the front of which the driver was seated. An innovative feature was the departure from the concept of the box tank with its single space into which all accessories were crammed. The Mark VIII was compartimentalised with a separate engine room at the back. This vastly improved fighting conditions as a bulkhead protected the crew against the deafening engine noise, noxious fumes and heat.
There were no machine guns in the sponsons, only the 6-pounders each manned by a gunner and loader. The side machine guns were to the rear of the sponsons mounted in the hull doors. Major Alden had designed the sponsons to be retractable (they could be swung in at the rear by the crew, being pivoted at the front), to reduce the width of the vehicle if enemy obstacles were encountered. Five more machine guns were in the superstructure: two at the front—left and right next to the driver—and one on each of the other sides. As there was no machine gun position covering the back of the tank there was a dead angle vulnerable to infantry attack. To solve this problem a triangular steel deflector plate was attached. The rear superstructure machine gunner could use it to deflect his fire down into that area behind the tank. The tank carried 208 shells and 13,848 machine gun rounds, mostly in a large ammunition locker in the centre which formed a platform on which the commander stood behind the driver observing the battlefield through a cupola with four vision slits. Later the side superstructure guns were removed on US tanks.
The twelfth crew member was the mechanic, seated next to the 300 hp Liberty V-12 (or in British tanks Ricardo V-12) petrol engine) cooled by a large horizontal radiator. Three armoured fuel tanks at the rear held 200 Imperial gallons (240 US gallons, or 909 litres) of fuel giving a range of 89 km. The transmission used a planetary gearbox giving two speeds in either forward or reverse. Top speed was 5.25 mph (8 km/h).
To improve its trench crossing ability to 4.88 m the vehicle had a very elongated shape. The track length was 34 ft 2 in (10.42 m) but even though the hull width was an impressive nominal 3.76 m, the actual length-width ratio of the tracks was very poor as that width included the sponsons. Combined with wide tracks it proved difficult to turn the tank. During testing many tracks twisted and broke in a turn and it was decided to use longer, stronger 13.25 inch (337 mm) links made of hardened cast armour plate, stiffened by webs formed by recesses in the track plate. Another effect of the narrow hull was that the fighting compartment was also very narrow. This was made worse by the fact that now the gap between the double track frames at each side was very wide; earlier types had only the tracks themselves widened. Nevertheless, the tank was supposed to accommodate another twenty infantry men in full gear if necessary. In absolute terms the vehicle was very large: at 10 ft 3 in (3.13 m) tall the Mark VIII was the second largest operational tank in history, after the Char 2C. However its weight was only 38.3 long tons (38.9 t) fitted for battle as the armour plate was thin with a thickness of 16 mm on the front and sides—a slight improvement over the Mark V but very thin by later standards. The roof and bottom of the hull were protected by only 6 mm thick armour plate, leaving the tank very vulnerable to mortar shells and landmines.
Mark VIII tank