Take this with a grain of salt, but I had a private twitter chat with a former Slant Six employee who was let go a couple years ago when the company closed. According to him, Umbrella Corps has actually been in development since 2012, and started as Operation: Terrestrial Fear.
Remember a few years ago there was a leak about Resident Evil Revelations coming to PS3/360/Wii U, but in that same leak, there was information about Operation Terrestrial Fear? Terrestrial Fear was supposed to have launched as a PS3/360 title, but after the leak, absolutely nothing was known. Well, this was because Slant Six closed unexpectedly to Capcom, so it was never revealed or confirmed. However, the game wasn’t killed.
After Slant Six closed in June 2013, this came unexpected to Capcom. However, Capcom took over development of Terrestrial Fear, transferring the entire development to their Osaka Studio. It is unknown why it took this long for the game to be revealed, but when it was revealed, the former employee noticed many changes to the game from their version.
The entire singleplayer storyline has been cut it seems, with only the multiplayer mode having been salvaged. What has been shown so far, the modes, maps, gameplay, etc, were all part of Terrestrial Fears online mode. The singleplayer however, was supposed to be about the Umbrella Team heading into the old umbrella facility on an abandoned Rockford Island (Code Veronica), and extracting traces of the virus from the ruins of the facility. Unlike Operation Raccoon City, terrestrial fear was to have been Canon, and would incorporate many story details from other RE games. Some things were left over however. Many of the zombie models, the maps, and the basic core summary of the soldiers heading into old facilities to retrieve samples of Umbrellas viruses.
Operation Terrestrial Fear was intended to be released only in Japan. (Despite Slant Six being Canadian) due to the controversy ORC caused (Bad Reviews, Low Sales). But apparently in Japan it did better. It was supposed to be launched on PS3 and Xbox 360. Now it seems its only being launched on PS4 and PC, but now it is being launched in America as well. The employees best guess is that the reason its being launched in America is because they are dropping Resident Evil branding from the title and making it stand alone.
He didn’t reply anymore after that, but did wish for me not to reveal who he was.
Again, take it with a very big (mountainous) grain of salt.