For decades the United States Army had relied on the M39 series 5-ton trucks as a primary heavy-hauler introduced during the 1950s and the 5-ton inventory was broadened some during the 1970s when the M809 was taken into service. By the end of the decade, thought had turned to an improved version of the M809 which AM General developed as the XM939 - to become the M939 in formal service. A pair of pilot vehicles were completed for evaluation and service introduction occurred in 1982. Since then, the line has grown through many variants evolved from the same base design.
The basic hauler utilized a 6x6, fully-suspended wheel scheme and was powered by a Cummins model NHC 250 series 6-cylinder diesel engine of 240 horsepower coupled to a 5-speed transmission system. Overall weight of the vehicle was 22,750 pounds while dimensions included a length of 25.6 feet, a width of 8 feet and a height of 9.6 feet. Road speeds could reach 65 miles per hour with a range out to 350 miles. The overall arrangement of the truck was conventional with the engine at front, the driving cab immediately behind and a flatbed section over the rear customized to suit a particular battlefield role.
AM General was issued an initial production contract in 1981 for 11,394 of the trucks. For a time, M939 trucks were being manufactured concurrently with the M809s they eventually succeeded. Early M939 examples were built from existing M809 vehicle stocks as AM General was responsible for their manufacture as well (unlike the M39 which arrived from International Harvester, Kaiser and others). The primary M939 and M939A1 trucks were by AM General proper while the subsequent M939A2 model (with Cummins 6CTA8.3 diesel engines) came from Bowen-McLaughlin-York (BMY).
The series eventually grew to include a slew of models evolved from the basic truck form. This included cargo haulers, a dump truck, a tractor truck (for trailer hauling), an expandable bed truck (a housing fitted over the flatbed rear) and a "wrecker" featuring a heavy-duty winch set over the rear. Even within some of these marks came sub-variants as in the M927 born from the family of cargo haulers - the M923 to M928 line. M939s have also been modified for active combat zone service like those serving in Iraq - showcasing "up-armored" cabs for some protection against small arms fire and IEDs.
The M939 family has gone on to see considerable export to American allies (either current or former) including Argentina, Egypt, Indonesia, Iraq, Mexico, Philippines, Saudi Arabia, and Turkey.