While light tanks have lessened their presence on the modern battlefield over time, the US Army began looking at adding such a vehicle to their stable in an effort to replace the outgoing M551 Sheridan "air-droppable" tank system for its airborne divisions. The type would combine speed and firepower in a complete package that could be airlifted by current aircraft within the Army inventory and set down by awaiting units operating near the front lines. Since airborne personnel were generally fielded with light weapons, the addition of a tank-killing system would have provided for a tactical advantage in-the-field. The primary mover of such a tank vehicle would be the venerable Lockheed C-130 Hercules with its exceptional hauling capabilities. The design of the light tank stemmed from the US Army's "Armored Gun System" program enacted in 1992 and, of the submissions reviewed, a joint design by the concerns FMC and United Defense was chosen for evaluation (part of the BAe Systems Land and Armament family).
The developmental vehicle was assigned the designation of "XM8" and usually written in full as "XM8 AGS" with the intent of production models designated simply as "M8". The type was built upon a conventional tracked chassis sporting six medium-sized road wheels to a track side with the drive sprocket at the rear and the track idler at the front (there were no track return rollers). The hull included a well-sloped glacis plate with a shallow hull superstructure and a raised engine compartment sitting atop a hydropneumatic suspension system. At the center of the hull roof was a turret sporting well-sloped sides for basic ballistics protection. A 105mm main gun was housed in the traversing turret which allowed for 360-degree engagement of targets. Power would come from a Detroit Diesel Corporation 6V-92TIA series diesel-fueled engine which could provide between 550 and 580 horsepower output. This equated to operational speeds of 45 miles per hour on roads (less off road) and an operational range of approximately 280 miles. As a compact light tank design, the system weighed in at under 19 tons with its base level armor protection scheme and was crew by only three personnel made up of the driver, tank commander and gunner. The driver was situated at the front center of the hull with the commander and gunner in the turret. Overall dimensions included a running length of 8.9 meters, a width of 2.69 meters and a height of 2.55 meters.
Primary armament was the developmental 105mm XM35 rifled main gun of which the XM8 carried 30 projectiles for firing. The weapon was centrally located at the front turret facing with overhang across the glacis plate when set forward. As the XM8 lacked a dedicated loader in the turret (or internal space of additional crew), the tank would have fielded an autoloading ammunition system providing up to twelve rounds-per-minute rate-of-fire. As a 105mm caliber weapon, the XM8 would have had enough hitting power to tackle armored targets save for the latest generation of heavily armored main battle tanks. Suspected projectile types would include high-explosive (HE) and armor-defeating rounds for tactical flexibility. Secondary armament was 1 x 7.62mm general purpose machine gun in a coaxial fitting next to the main gun armament and managed by the gunner. On the turret roof was an optional 12.7mm M2 Browning heavy machine gun to counter low-flying aerial threats as well as light-armored vehicles. 4,500 rounds of 7.62mm ammunition were carried aboard as were 210 rounds of 12.7mm ammunition for each machine gun respectively. Twelve smoke grenade dischargers (eight to the right turret side and a further four to the left) were included for self-generating smoke screens to protect the tanks movements in offensive and defensive maneuvers.
It was intended to showcase the XM8 design with three levels of possible armor protection (all including titanium). "Level I" was the base protection scheme offered which kept the vehicle's weight at 19.25 tons and allowed the tank to retain its "air-droppable" insertion quality. "Level II" provided for more crew protection at the expense of additional weight (22.25 tons overall) and losing the air-droppable quality. "Level III" protection offered the maximum armor protection for the crew while losing its C-130 transportation quality, instead having to be transported via the heavier US military breeds. However, this level of protection was deemed suitable against anti-tank weapons that posed serious threats to light tank-classified weapon systems so the restriction was quite negligible.
Despite the promising nature of the XM8 project, the US Army decided against replacing their M551 Sheridans in inventory. As such, the program was cancelled after six prototypes had been completed. It was expected that the XM8 would enter production as the M8 in 1996 but the program's cancellation negated such work. Regardless, the M8 is still a viable combat system being offered by United Defense to interested outside parties - of which only a few have shown any interest in and none have placed any procurement orders. The lack of interest on the part of the US Army has definitely hurt the chances of the XM8 being accepted anywhere else in the world.
In September of 2006, United Defense unveiled the "Thunderbolt 120mm" demonstrator which was essentially the XM8 with improved capabilities including an autoloading XM291 120mm main gun, computerized fire control system, hybrid powerpack, improved armor protection and standardized FLIR night-vision. The additions have not forced the XM8 to lose its air-droppable quality, making it a tempting solution to the discerning customer.
The M551 Sheridan series, itself, was retired from US Army service in 1996, having been in service since 1969 and seen combat action in the Vietnam War, Operation Just Cause and in Operation Desert Storm. A viable alternative ended up becoming the Stryker series of 8x8 wheeled armored fighting vehicles.