A few years back Delta tweeted a youtube video of Arnie explaining the process. It's deleted now https://www.youtube.com/watch?v=l0EeBvEwtnc.
The process is getting a trigger to get your app scored, where two from the hiring department sit down with your app, and give you a score based on whether you meet the motivation required to be a Delta pilot. So in other words there are no real requirements other than the minimums to apply.