Overwatch always opens in windowed mode?

I've found people who have had a similar issue here. The solution (temporarily until Blizzard sort it out) seems to be:

1. Go to your Overwatch game settings (Documents -> Overwatch -> Settings).
2. Delete your settings file.
3. Launch the game, change settings to full screen.
4. Go back to your settings file, right click and set it to read only.