In the late 1960s, Duplex introduced the R300 custom chassis for fire apparatus. Duplex changed the designation to the D300 in 1979.
The D300 was cab forward 2 axle chassis with a TCM 5-person "Cincinnati" contour cab. The chassis was intended to be used for pumpers and similar sized apparatus. Duplex ended production of the D300 chassis in the early 1980s.