Ferrari only launched its Special Projects division a decade ago, but its history of crafting one-off vehicles for discerning and extremely wealthy customers goes back much further than that. For example, just look to the auction block in Arizona today, when RM Sotheby's will sell off this timelessly elegant 250 GT Coupe Speciale from 1957.

The one-of-a-kind Prancing Horse was built for Princess Lilian de Réthy. The wife of King Leopold of Belgium might have been named Queen if not for having been born a commoner, but was evidently treated as the royalty she was by the Italian automaker.

Far from just a moneyed customer, Princess Lilian helped Ferrari at a critical time, when Pirelli suddenly dropped out as the Scuderia's tire supplier. Lilian arranged for Belgian rubber company Englebert to take Pirelli's place, cementing a longstanding relationship between the three.

Two years later, Leopold and Lilian ordered this special 250 GT to be coachbuilt by Pinin Farina to their specifications – one of many they'd commission over the years. It borrowed elements from the Tour de France, the California Spider, the Series I Cabriolet, and set the stage for the 250 GT Pinin Farina coupe to follow.

The princess took delivery at the royal estate in Waterloo in 1958, and drove it for ten years on diplomatic plates. But after taking delivery of another (based on the newer 330 GTC), she gifted this one to an American doctor. The car has traded hands many times since, appearing in an endless string of concours, books, and magazine features along the way. But this will mark the first time it will be offered for sale in 20 years. RM Sotheby's expects it will garner around $11-13 million when the gavel drops, potentially setting the bar for the auction year ahead.