According to Wikipedia, with various sources, allied forces in Italy would often ask for bacon, eggs, and cheese on noodles, called "spaghetti breakfast," so Italian chefs would modify the existing recipe for "pasta cacio e uova," which was originally without meat, to feature cured pork, thus creating the original carbonara.

I didn't see anything to specifically say whether they originally used bacon as the allied forces asked, or used other more traditional forms of cured pork from the start, but now guanciale, a cured pork jowl, is considered the traditional ingredient, though bacon is a common substitute outside of Italy.