The Mack NJU 5- to 6-ton 4x4 Ponton tractor (G639) was a semi-tractor designed to haul bridging equipment during World War II.

In 1940 the US Army ordered 700 Mack 4 x 4 truck tractors, intended to tow pontoon-carrying semi-trailers.

In November 1941 119 semi-tractors were delivered to the British army in Egypt, where they bore War Department H-numbers.

[2] The design was a militarized version of a civilian Cab Over Engine (COE) model, partly redesigned to make it 4WD.

A Mack engine and transmission were matched with a Timken 2-speed transfer case and double-reduction axles.