I applied for it last year. It just depends on if Investigators like your letter, cv and record. I think they cut it off March 15 as the last date Investigators can contact you.
Good luck! NIH is an awesome place to work at, and you can't beat living in DC
You will get contacted by the PI based on your interests and experience. I don't believe you need stellar stats for NIH summer internship, just convey strong enthusiasm and commitment. I would suggest waiting to hear from a few PIs before scheduling a trip to Bethesda or whichever off campus site to visit + interview unless you live close.