Captain Jack (Updated: 6/2/2016):
Paratroopers, in general, were more or less lightly-armed infantry soldiers dropped via parachute from passing transport aircraft. As they were often limited in what they could carry into battle, they would most often times be outmatched when facing off against a "regular" army force equipped with artillery, mortars, machine guns and tanks. As such, every tool in the arsenal of the paratrooper had to be deemed essentially to his operation and was specifically selected for its usefulness in combat. Airborne troops themselves were developed for light, temporary engagements - not prolonged warfare, particularly against enemy armor. World War 2's battlefields would often push the boundaries of what airborne elements were capable of - and provide for a unique assortment of answers as the battle waged on. Notable Allied airborne forces of the war became the American 82nd and 101st and the British Red Devils of the 1st and 6th.
It was British authorities that first called for such an air-transportable tank solution. As their own wartime resources were being pushed to the brink, it was the Americans and their massive war industry output that was called into play to help support the Allied cause. Despite a surplus of British light tank units available, these were deemed obsolete with the changing face of war and, furthermore, were never specifically designed for airborne use all the while holding inherent limitations all their own. In particular, the Mk VII Tetrarch Light Tank was of note, though its best years were clearly behind it. Design and development of the British General Aircraft Hamilcar glider was underway and testing using the Tetrarch proved the concept of an air-transportable tank viable. Now an initiative was forged to design a develop a reliable light tank for airborne personnel.
A set of specifications was formally made available in February of 1941, calling for a 10-ton maximum, tracked, armored vehicle mounting a 37mm main gun and crewed by three personnel. Self-defense would come in the form of .30-06 Browning M1919A4 machine guns. Armor protection would have to meet the requirement of 30mm to 50mm in thickness. Range was expected to be in the vicinity of 200 miles with a top road speed equal to 40 miles per hour. The tank would have to be agile in nature, fast and quick-reacting to take on the enemy and counter its movements while supporting allied airborne infantry in the process. Its tracked nature would ensure that the type could be used to good effect off-road as well as on-road. The United States Ordnance Department was charged with the light tank's design and development. Among the respondents of the initiative were the concerns of General Motors, J. Walter Christie and Marmon-Herrington Corporation. In May of 1941, the Marmon-Herrington proposal was formally selected by the USOD to which a prototype then appeared before the end of the year. By this time, the prototype had been assigned the designation of "Light Tank T9 (Airborne)".
After some early evaluation, the T9 pilot vehicle was found to be lacking key areas. While sporting an-all welded hull and powered cast turret, it was too heavy for the intended role. Wheels were deemed too weak for the weight distribution required of the design and the expected rigors of combat use leading to the installation of steel beams to reinforce the suspension system. Armor protection was a long-term concern but the T9 pilot vehicle was already reaching its maximum allowable weight for use in the Hamilcar glider. As such, another form - the T9E1 - was developed with a new manually-powered turret (the intended gun stabilizer was dropped) and a revised, sloped hull design. A pair of .30 caliber machine guns fitted to the bow were now dropped to save weight. The suspension system was further revised to be made lighter and brought the T9E1 down to manageable levels for glider use.