The Boulevardier is a classic IBA (International Bartender Association) cocktail.
The recipe was first published in 1927 in the book Harry McElhone's Barflies and Cocktails, in an article on page 80 where Harry McElhone attributes the invention of the cocktail to Erskinne Gwynne.
In this article, the original recipe was then: 1/3 Campari, 1/3 Italian vermouth, 1/3 bourbon whiskey. But this recipe lacks balance, the proportions are now 3/2/2.
Note that if you replace the Bourbon with gin you get the Negroni cocktail.