In the late 1960s Duplex introduced the R400 custom chassis for fire apparatus. Duplex changed the designation to D400 in 1979.
The D400 was a cab forward 3 axle chassis with a TCM 5 man "Cincinnati" contour cab. The tandem axle chassis was intended to be used for aerial ladder trucks and similar large apparatus. Duplex ended production of the D400 chassis in the early 1980s.