The 321-series conventional "L" models and forward control "LP" models entered production in 1957. These new vehicles initially had a payload capacity of 5.5 tonnes and were available as flatbeds, tippers, and articulated tractor units. As with the majority of the range, all wheel drive was an option. Power came from the 5.1 litre six cylinder diesel OM 321 engine which produced 110 hp at 3000 rpm.
The forward control variant used the same design of cab which was introduced three years previously in 1954 with the LP 315 model. The biggest differentiation between the two model's cabs was the length behind the front seats. The LP 315 had two extra seats in the rear which could be converted into a bed, whereas the LP 321 utilised what is now known as a day cab, as it was only a medium duty truck which drivers would not need to be in for any longer than a day at a time.
The L 321 was available with either a 3.6, 4.2 or 4.8 metre wheelbase, and the LK 321 tipper variant's wheelbases were either 3.2 or 3.6 metres. The forward control LP 321 version was available with all four previously mentioned wheelbases. Payload capacity depended on wheelbase and ranged from 5.4 to 5.7 tonnes for the bonneted variants, and 5.8 to 6.3 for the cab over engine versions. Production of the 321 series only lasted two years until it ceased in 1959.