The 4x4 scout car M3A1 was the last in the line of descendents of the scout car M2. The M2 had been based on the Corbitt 1.5 ton truck, was powered by Lycoming's Corbitt Eight 94 horsepower V8 engine, weighed 7,900lb (3600kg), and could travel at 50mph (80kph). It was armored with .5" (1.3cm) plates on the front and .25" (.64cm) plates on the sides and rear. Twenty-two M2s, including prototypes, were produced in 1937-8.

The scout car M2A1 was manufactured by the White Motor Company. It featured tourelle skate mounts for its machine guns instead of the external pintle mounts found on the M2. It was powered by the Hercules JXD engine that made 95 gross horsepower at 3000rpm and 224 gross ft-lb of torque at 1100 rpm, and could be propelled to 60mph (100kph). The M2A1 was 202.50" (514.35cm) long, 80.37" (204.1cm) wide, and 81" (206cm) tall. Front and rear tread were the same at 63.75" (161.9cm). It could hold eight men. Its minimum turning circle diameter was 29.25' (8.915m), ground clearance was 9.75" (24.8cm), maximum fording depth was 22.50" (57.15cm), and ground pressure was 57psi (4.0kg/cm²). The designation of M2A1 was changed to M3, and 100 M2A1/M3s were produced.

The M3A1 sported a wider body than the M3 with square corners and an overhung top, and its machine gun skate rail was lowered below the level of the armor. The rear door found on the M3 was deleted, and stowage and seating was changed. An anti-ditching roller was also added. A detachable canvas top was provided for the passenger compartment; it was supported by three bows and the windshield frame. The compression ratio in the JXD engine was raised to 5.88 from the M3's 5.78, yielding more power (110 gross horsepower at 3000 rpm and 241 gross ft-lb of torque at 1100 rpm).