In a new QA on PlayStation Blog, Epic Games has shared that the first prototype ideas for the Battle Royale mode in Fortnite was being worked on in July 2017, and the company then released the first playable version of the mode in September 2017 — two months of development time.
In the past, the Fortnite team had been experimenting with a number of PvP modes, but ultimately decided to move its focus to the PvE experience. There was always the intention to go back to that. Last year, the idea of a battle royale-style game came up. We just thought it would be cool to combine the last-man standing gameplay with the action-building mechanics that make Fortnite unique. In July of last year, we started to work on a prototype, and then released our first version of the game in September. So, roughly two months. There was such a good, solid foundation there for us, so that certainly helped us get to a playable version so quickly.
The company did state, and continues to say, that the BR mode is in ‘Early Access’ as they have released updates to the game almost weekly since its initial launch in September 2017.
SOURCE: PS Blog