When you think of American luxury cars, you probably think of Cadillac. Since its inception, in 1902, by founders William Murphy, Lemuel Bowen, and Henry M. Leland, the brand has been synonymous with lavishing luxury and style. Moreover, on a few occasions, Cadillac designers played with the idea of radical designs and gigantic (even by American standards) engines. Thus cars like the XP-840 Eldorado Fastback spawned.

The Cadillac XP-840 Eldorado Fastback was one of those obscure projects that inspired future models. It was approved in 1963 and was one of the first 1960s Cadillac concepts. Being substantial was always part of Cadillac’s philosophy when designing a car and not many cars embodied that as good as the XP840.

According to Jordan, this was a "pet project". In 1965, the project progressed to a full-size mockup, after the design achieved “official” status as one of GM’s experimental projects, hence the XP840 designation.

The car was meant to be a V-16 two-seater, which is why the front end was unnaturally long. Data on the engine is scarce, but it is believed that the engine would have been made of two V-8 engines, used in the seventh-generation Series 62 Cadillac models. Those would be the 6.4-liter (390 cu in) or 7.0-liter (429 cu in) OHV V-8 engines.

The XP840 Eldorado Fastback featured many “firsts” like the lack of a rear window. Instead, a small opening was made for a rear-facing TV camera. The windshield was V-shaped and continued all the way to the rear section of the canopy. The double-notched beltline and rear panel, which were “hidden” between the rear fender lines ended up being some of Cadillac’s distinctive design elements.

It’s hardly a surprise, many design cues from the XP-840 made it on future models, first of which the 1967 Eldorado. Like many other daring designs, the XP-840 never made it to production, but its legacy carried on into other production models. In reality, Cadillac did not have a need for a V-12 or a V-16 car and the project was mostly for fun, even if they made another V-16 concept many years later, in the Cadillac Sixteen.