J. Walter Christie, an American automobile design, attempted to sell the United States Army on various track-and-wheeled vehicles since the latter half of World War 1 (1914-1918). The Army provided a contract in 1919 to cover the Model 1919 Chritstie Medium Tank, a "convertible tank" with hybrid suspension system allowing it to be run on roadwheels or linked-track sections. The Model 1919 was not an outright winner as it was recognized to be underpowered, unreliable and an uncomfortable ride for its two-crew.
Rather than wait on the formal rejection by the Ordnance Department, Christie requested additional time to rework his creation and this was granted. In about a year, Christie returned with a revised version of his Model 1919 and this became the Model 1921. The tank retained its large roadwheels at the extreme corners of the design and held a single two-wheeled bogie at center of each hull side. However, the front wheels were now sprung for ride comfort and the hull superstructure completely reworked. Gone was the turreted main and secondary armament. In its place was a fixed barbette mounting the 2.24" main gun. To either front corner of the hull structure were ball mounts, each fitting a 0.30 caliber air-cooled machine gun.
Gone was the tactical flexibility of mounting both armament in traversing turrets and the Model 1921 did little to improve on performance and reliability. U.S. Army authorities were not sold on the design and the Model 1921 joined its Model 1919 forerunner in not being pursued. However, it was tested into July 1924 and a Bankrupt Christie eventually sold off rights to these machines (and some applicable patents) to the Army.