I unlocked this ($100,000,000 needed, not $25,000,000) once i ended my first season as you recieve a payout - for me it was around $87.4 million.
Once i started my second, and picked my major sponser again (you can choose different major sponsers and power units), the achievement popped up on my screen
I also do not think it matters the distance you do, or your AI level, as i am doing 5 laps compared to my housemate who is doing 100% and we are relatively the same ^-^
Edit: My housemate got this achievement BEFORE he ended his first season - he was 3/4 of the way through his first season before starting again, so this must roll over if you start again. He also has high paying sponsers whereas i go for ones that look cool, so keep that in mind :D