First attempts by Daimler-Benz to assemble a 6-cylinder diesel engine in a Mercedes-Benz Mannheim chassis ultimatively failed due to its enormous vibration.
170 pullman-landaulets based on the W21 chassis were built from 1936 to 1937, called the Nullserie and used only as taxis, with a three-speed plus overdrive transmission, without syncromesh on the first gear.