Sometime in 1943, a project was underway to investigate how far light tank firepower could be improved. This culminated with the installation of a 75 mm gun from the M4 Sherman into the 75 mm HMC M8, which originally held a howitzer for infantry support. The modification proved successful, to the extent that both Armored Board and the Tank Destroyer Branch requested the conversion for evaluations. The modified M8, considered as the 75 mm GMC M8 or the "M8A1" in the evaluations, was determined to be a sufficient modification, carrying and firing the gun without detriment to the vehicle, with the downsides being the expedient nature of the modification, small ammo stowage, and crew discomfort due to the smaller internal space. While Armored Board had no interest in procuring the vehicle, the Tank Destroyer Branch found the M8 GMC suitable as an expedient design prior to the fielding of their newest M18 Hellcat tank destroyer, but this ultimately went nowhere when it was determined that the M8A1 could not be produced any sooner than the projected M18, and so the vehicle project was ended by June 1943.

Introduced in Update 1.45 "Steel Generals", the M8A1 is a Stuart light tank package with a medium tank firepower not too unlike the M24 Chaffee. Although its armour and crew survivability is poor, the emphasis on firepower and mobility allows players to use it as a light tank destroyer to flank and destroy enemy tanks with its firepower. Though the open-top design makes the vehicle vulnerable to aircraft strafing, a M2 machine gun is available to try and fight back against enemies, hopefully warding them off enough to avoid outright destroying the M8A1.

The M8A1 is one of the rarer American vehicles to acquire, being made available in a Winter 2014 event as an unlockable gift in the "War Thunder Christmas Special". After that, the M8A1 became available again as a Warbond shop reward as part of War Thunder's 4th anniversary.