A 20 June 1945 report by the Army Ground Forces Equipment Review Board recommended that all 4‑ton to 6‑ton tactical trucks should be replaced by a single standard 5‑ton (4,536 kg) 6x6 truck series. In 1949 specifications were set and truck manufactures began working on prototypes. Chrysler, GMC, and Mack’s designs were advanced, International Harvester’s was a conservative conventional, similar in size and layout to the earlier 6-ton (G512) series.
The International Harvester design was chosen and rushed into production in January 1951, it would be standardized as the M39 series. Kaiser (renamed Kaiser-Jeep in 1963) also became a major manufacturer, with Diamond T and Mack building smaller numbers. In 1963 Kaiser-Jeep began building the final order, production was completed in 1965.
In the 1960s more trucks were required, and the Army wanted to replace the multifuel engines with a standard diesel. AM General (successor of Kaiser-Jeep) developed an updated and redesigned version of the M39 series. Standardized as the M809 series, the primary difference was the engine. The hood, frame, and fenders were lengthened to make room for the larger engine, and it had a redesigned grille. All had an air cleaner on the left front fender, a quick visual way to tell them from the earlier M39 series. Jeep/AM General built all M809s between 1969 and 1982.
The M939 series was a Product Improvement Package of the M809, with updated engine, transmission, and brakes. A new, larger cab and tilt-forward hood were a major visual change from earlier trucks. Early M939s were rebuilds of M809 vehicles, suffix –A2 are new production by Bowen-McLaughlin-York/BMY with later model Cummins engine.
The M39 series were originally powered by a Continental R6602, a 224 hp (167 kW) 602 cu in (9.9 L) inline 6 cylinder gasoline engine. The -A1 upgrade had a Mack ENDT-673, a 210 hp (160 kW) 673 cu in (11.0 L) turbocharged inline 6 cylinder diesel engine. The -A2 had a Continental LDS-465-1A 175 hp (130 kW) 478 cu in (7.8 L) turbocharged inline 6 cylinder multifuel engine.
The M809 series used a Cummins NH250 engine, a 240 hp (180 kW) 855 cu in (14.0 L) naturally aspirated inline 6 cylinder diesel engine. The M939 and M939A1 models used the NHC 250 engine, the M939A2 models use a modern Cummins 6CTA8.3 240 hp (180 kW) 504 cu in (8.3 L) turbocharged inline 6 cylinder diesel engine.
The M39 and M809 series had a Spicer 5 speed manual syncromesh transmission. The M939 used an Allison automatic, for better engine speed control and driving ease.
A two speed transfer case also engaged the front axle. M39s and M809s used one which engaged the front axle automatically if the rear wheels turned faster than the front, as when the rear wheels spun. The M939s had an improved type, which always engaged the front axle in the low range, in the high range the driver could engage and disengage it with an air control.
A ladder frame with three live beam axles, the front on leaf springs, the rear tandem on leaf springs with locating arms. Brakes on the M39 and M809 were air over hydraulic with drum brakes on all wheels, M939s were full air. Many trucks were available with a front-mounted 20,000 lb (9,100 kg) capacity winch.
There were three wheelbases (Measurements are from the centerline of the front axle to the centerline of rear tandem). The short, used for tractors and dumps, was 167 inches (4.24 m), the long, used for cargo, wreckers, and bolsters, was 179 inches (4.55 m), and the extra long, used for long cargo, tractor wreckers, and expansible vans, was 215 inches (5.46 m).
Most models had 11.00x20 size tires with dual rear tires, bridge trucks and some chassis-cabs had 14.00x20 with dual rear tires. Early M939s used 11.00x20s with dual tires, but M939A1s had 14.00x20s with single rear tires and M939A2s introdued a central tire inflation system.
Cargo trucks had a 14 ft (4.3 m) long low sided box with a bottom hinged tailgate, bodies with drop sides were also standardized. Both had removable side racks with fold down troop seats and bows for a tarpaulin. Long cargo trucks, with an extra long wheelbase, had a 20 ft (6.1 m) long box with side racks and bows for a tarpaulin. There were no drop side versions, and none had troop seats.
Dump trucks were used to haul sand, gravel, dirt, rubble, scrap, and other bulk materials. They had a 5 cubic yards (3.8 m3) dump body with cab protector and a tailgate that could hinge at either the top or bottom. They could be equipped with overhead bows, a tarpaulin, and troop seats, but the relatively small size of the body limited their passenger or cargo load.
Medium wrecker trucks were used to recover disabled or stuck trucks and lift large components. A rotating, telescoping, and elevating hydraulic boom could lift a maximum of 20,000 lb (9,100 kg). Although the truck was not meant to carry a load, the boom could support 7,000 lb (3,200 kg) when towing.
Tractor trucks were used to tow semi-trailers up to 37,500 lb (17,000 kg) with 15,000 lb (6,800 kg) on their fifth wheel. Semi-tractor/trailers have to stay on relatively flat ground, and are not rated for full off-road use. On improved roads they could tow up to 55,000 lb (25,000 kg) with 25,000 lb (11,000 kg) on their fifth wheel. Tractor trucks normally towed a 12-ton 2 axle trailer.
Medium wrecker tractor trucks, with an extra long wheelbase, were a wrecker with a fifth wheel mounted behind the boom. Meant for aircraft recovery, the truck could perform wrecker duties and load and tow semi trailers.
Expansible van trucks had a 17 ft (5.2 m) long van body with a slide out section on each side. When the sections are extended the working floor was over 12 ft (3.7 m) wide. Some had hydraulic lift-gates.
Bridge transporting trucks had a stake body 20 ft (6.1 m) long for carrying bridging equipment and components. In the M939 series there were no standardized bridge models, instead specialized bodies were mounted on chassis-cabs.
Logging bolster trucks, with a bolster trailer, were used to carry long loads like logs, poles, and bridge sections. When unloaded the trailer could be loaded onto the truck. There were no bolster trucks in the M939 series.
Chassis cabs were produced in different wheelbases for specialty bodies. The largest, the M39 series M139C/D, was an Honest John rocket launcher.