Marvel Studios president Kevin Feige revealed that they always had back up plans for the Marvel Cinematic Universe's Phase Three in the event something changed, including a deal for Spider-Man rights with Sony.
In an interview with SlashFilm, Feige spoke a little about how the Phase Three schedule came about and how their contingency plans for Spider-Man factored into their decision making:
As you might have imagined, that had been in the works for a long, long, long, long time. And we had, as we often do, mapped out various alternatives if things don't happen. So we had a plan how it would continue without him and we have a plan how now it will continue with him. I wouldn't say it was "drastic" but we were certainly happy when it all came together.
Disney and Marvel Studios announced their plans for the MCU's Phase Three last October with a schedule that included "Captain America: Civil War," "Black Panther," "Captain Marvel" and "Inhumans" films. Only a few months later, Marvel and Sony struck a deal to share the rights to Spider-Man, meaning that Spider-Man could cross over with the Avengers and vice versa. Following the deal, several Marvel films -- "Black Panther," "Captain Marvel" and "Inhumans" -- were pushed back to make way for solo "Spider-Man" films.
The first solo installment of the MCU's "Spider-Man" is scheduled for release on July 28, 2017.