The first design prototype for the Scenicruiser, the GX1, was a double decker with access from the lower deck and the driver seated on the upper deck. It was soon decided that a split-level design would be better because the GX1 was too tall for many Greyhound garages and lacked luggage space for 50 people. The GX-2 had a lower level containing the driver's area and entrance with ten seats plus a restroom on the passenger's side and an upper level with 33 more seats. This arrangement also allowed a large baggage compartment underneath the second level. This design was called the GX-2. Both the GX1 and GX-2 were actually built by Greyhound from 1947 to 1949 with help from GMC. In late 1951, GMC started work on its first prototype, called the EXP 331. It was completed in 1954 and had some unique features that were not used on the production versions. After the last PD 4501 prototype was built, it was rebuilt as a production model with serial number PD 4501-1001. The Scenicruiser was equipped with air-ride suspension and air conditioning. The coaches were unusual in having ten wheels. Each of the two rear axles had four wheels but only the forward axle was powered.