Simon-Duplex introduced the D500 in the late 1980s. The chassis received the name Vanguard and came standard with a 6-person aluminum canopy cab. A four door enclosed cab was an option seating up to 8 firefighters. An optional raised roof section over the rear compartments was available.
The D500 had a split cab, the forward section tilted to provide access to the engine, while the rear section remained fixed to the chassis. In 1991 the enclosed cab became standard to comply with the new NFPA standards.
The D500 was a 2 axle chassis intended for pumpers and similar size apparatus. The chassis was available until the late 1990s.