In the early sixties, the small basic Unimog alone was no longer capable of meeting the growing demands and covering the ever more diversified range of assignments. The Unimog S, first and foremost a military vehicle, was not always the right alternative for civilian applications, even though it was available in finishes other than olive green. Accordingly the company launched a medium-heavy series with the designation 406 in 1963. The wheelbase of this new, additional Mercedes-Benz Unimog was 2380 millimetres.
At the same time, larger diesel engines were for the first time installed under the Unimog's short bonnet: 65 hp generated from four cylinders and a little later even from the legendary, large-volume OM 312 six-cylinder diesel engine with a displacement of 5.7 litres – setting a new standard in performance. With reference to its hp rating, the new Unimog became known as the U 65. With the additional 406 series, Daimler-Benz complemented the Unimog range by a genuine jack-of-all-trades for use on and off the road, permitting completely new applications, for instance as a versatile tractor.