I don't know why US offers so many very tight connections through CLT, though they have accommodated me in the past by changing my incoming flight.

40 mins will be tight. If your first leg is ontime then you will be fine, but if you're delayed then you have a problem of course. If your schedule is flexible then I wouldn't worry about it - they'll get you on another flight if possible - but if it's important that you make it, I would change it or go standby for an earlier flight that day.

Good luck.