Sakurai - Day one development of Project Sora involved Kid Icarus

"I developed a concept for a hybrid between an aerial shooter and a ground-based third-person shooter. Exploring that and talking with [Nintendo president Satoru] Iwata, we realized that Pit would be a good match for this. Not only Pit's abilities and personality as a character, but also the entire world of Kid Icarus would fit well, too. So, from the very first day of development, we had the intent of this being a Kid Icarus game." - Masahiro Sakurai

While the idea Sakurai had didn't involve the Kid Icarus license, the game's actual development didn't begin until Kid Icarus was already cemented as a design decision.

Full interview here
Series retrospective here