From the 1950s through the late 1960s, top executives at Chrysler, Ford, and GM commissioned their respective design studios to create custom rides for their personal transportation. One of those executives was Bill Mitchell, who had come to power in 1958 as Vice President of General Motors Design Staff. Mitchell had replaced the legendary Harley Earl and was instrumental in the fresh new look of GM products throughout the 1960s.

Mitchell had liked the redesign of GM's fullsize models in 1961 and ordered the design staff to produce a 1962 Grand Prix convertible, dubbed the X-400, for him to drive when he wasn't tooling around in his Sting Ray. This wasn't the first Pontiac X-400; both a 1959 and 1960 version had been built based on the Bonneville. Since there was no convertible Grand Prix in the Pontiac product line, a blazing red Catalina ragtop was built specially for Mitchell, and then sent to design staff studios in the GM Tech Center to be converted into a Grand Prix and to add the finishing touches.

The X-400 sported a sparkling red interior and white convertible top, but it wasn't just a pretty face. It packed a solid punch under the hood. Mitchell loved speed and racing, and his Grand Prix was required to kick asphalt.

On the outside, the X-400 was mildly customized. The handsome Grand Prix front end and split grille theme was retained; however, the standard headlamps were replaced by rectangular, single-bulb Cibie Marchal headlamps. Swing-up, mesh grille stone guards covered the Cibies and a unique chrome ornament was mounted on the front centerline of the hood. Also on the hood were two functional grilles, one on each side, with the word "supercharged" boldly announced in chrome block letters.

On the flanks were brushed aluminum concave moldings, and dual exhausts exited through twin ports at the rear of the quarter-panels, just ahead of the back bumper. Pontiac's unique eight-lug aluminum wheels were mounted on whitewall tires. The fabric top stack was covered by a fiberglass top boot, and body-colored Talbot-style outside rearview mirrors graced each door.

The interior was also mildly customized with red carpeting, red leather thin-shell racing-style bucket seats, a padded console between the seats, and special brushed-aluminum door panels with red leather padded inserts. Facing the driver was a special wood-rimmed steering wheel with three spokes, and a special "Grand Prix X-400" insignia in the horn button. The upper and lower dash was padded and a bank of three gauges, canted towards the driver, replaced the radio. On the left was an 8,000-rpm tachometer with a 5,500 redline. In the middle was a Judson-style supercharger boost pressure and vacuum gauge, and the righthand gauge was a clock.

A specially-built console housed a glovebox and a removable "Trans Portable" AM radio. Below that was a Muncie shifter that stirred a Warner T-10 four-speed manual transmission. Next to the shifter was another lever that controlled the loudness of the exhaust. It was a pet project of Pontiac Chief Engineer John DeLorean, and would eventually be known as the "Tiger Button." (This would eventually evolve into the short-lived vacuum operated exhaust option on the 1970 GTO.)

Under the hood was the new Pontiac 405hp 421ci engine, specially modified by Mickey Thompson with some of the same tricks he was using in his Pontiac land speed attempt vehicles. The chrome-trimmed 421 was topped by a 4-71 GMC blower with four 1954 Corvette Carter sidedraft carburetors. The quartet of Carters was fed cold air through the open hood grilles. The "Positive Displacement" GMC huffer was set at 5.5 lbs boost and produced enough grunt to power the 4,700-lb X-400 down the quarter-mile to the tune of 14.92 seconds at 97.82 mph with Mickey Thompson at the wheel. That was just the kind of go-fast that made Bill Mitchell a very happy executive.