The M3 Scout Car was the main reconnaissance vehicle of the US Army at the start of World War II and would inspire the M2 and M3 family of half-tracks which were also designed by White. The M3 was based on the chassis of the company's own civilian trucks, modified to incorporate sufficient armor to protect against small-arms and shrapnel. Protective features included armored shutters on the radiator as well as an armored plate over the windshield. Another distinguishing feature was a roller mounted at the front of the vehicle which would assist the vehicle in crossing ditches; this would later be added to the M2 and M3 half-tracks as well. The troop compartment was open topped but could be fitted with a detachable canvas top if needed. Armament consisted of a single 12.7-mm M2HB mounted on a skate rail along with one or two 7.62-mm M1919A4 machine guns in the rear of the troop compartment. The M3 was produced in large numbers and served with US Army mechanized cavalry units during the first half of the war, where its high speed and mechanical reliability made it popular with its crews. However, poor off-road performance and lack of overhead protection was a liability and led to the M3 being eventually replaced by more heavily armored vehicles like the M8 and M20. Around half of all production was provided as Lend-Lease to Britain and the USSR as well as free French, Belgian, Czech, and Polish forces. Hundreds were subsequently sold off abroad with some still in service into the 1980s.

A requirement for a new mechanized cavalry vehicle resulted in prototypes by Marmon-Herrington and White, the latter which was known as the T7. These were standardized into the M1 and M2 by 1938 before the definitive M3. Only a small number of units were built before the definitive M3A1 which accounted for nearly the entirety of the production run. This had a lengthened and widened chassis, rearranged seating, and incorporated the roller. No other variants were built.