The M939 5-ton (10,000lb (4,500 kg)) 6x6 truck is an improved version of the M809 series. These improvements include a hydraulic powered front winch, an automatic transmission, complete air brake system, a 3-member crew cab, and a more efficient power steering system and electrical system. Production for the M939s started in 1982, by AM General with the first 11,000 as rebuilds of the M809 chassis. The M939A2s were manufactured by Bowen-McLaughlin-York (BMY) in 1989. The M939s are still in service, but since 1991, they are slowly being phased out by the Medium Tactical Vehicles (MTV).
The M939 series comes in 6 body variants which are; cargo, long wheel based cargo, dump, wrecker, and van. There are 3 wheelbases short, standard long and extra-long. The 14ft 11in (4.55m) “standard long wheeled” is used for cargo and wrecker variants. The 13ft 11in (4.24m) “short wheeled” is used for tractors and dump variants. The 17ft 11in (5.46m) “extra-long wheeled” is used for long cargo and expansible van variants. The M939 series has a ladder frame with 3 axles with the front axle on semi-elliptical leaf springs and rear tandem on quarter elliptical leaf springs. The brakes are full air which is an upgrade from the air over hydraulic drum brakes on the M809s.
All M939s are equipped with an Allison automatic transmission with an improved 2-speed transfer case that engages the front axle in low range. The driver can engage in high range with an air control. M939 and M939A1 use a Cummins NHC 250 240hp (180kW) 855 cu in (14.0l) naturally aspirated inline 6-cylinder diesel engine. M939A2 use a Cummins 6CTA8.3 240hp (180kW) 504 cu in (8.3L) turbocharged inline 6-cylinder diesel engine. The A2s received a central tire inflation system. The M939Als main upgrade was 14:00xR20 super-sized tires and rear tandem singles which also required modification to the spare tire rack and lifting device. Original M939s use 11:00xR20 tires and rear tandem duals. In 1991 till 2003, anti-lock brakes were added in response to the vehicle accidents caused by poor brake performance and stability when loaded.