The Model PD-4501 as GMC called it was the most distinctive American "parlor" bus design of the modern era. It was the result of seven years of effort by Greyhound and GM Truck and Coach Division. The first GX1 prototype was based on a design by Raymond Loewy as . Originally conceived as a bus, Greyhound later used a tandem-axle prototype by Loewy called the GX-2 to lobby for the lifting of length restrictions of buses longer than 35 feet in most states at the time.