Always the same story. We both agree to the boundaries of our new found friendship. Give it 2-3 months. Then she wants that "title".
If you really want a FWB, then don't try to get one. Women can sense when men only want the yambs. That's not really a deterrent for them. What detours women is how men approach that. If a woman thinks you only want the box (and you aren't famous,) chances are she's not going to be very receptive. However, if a woman thinks you want to legitimately spend time with her outside of box stuffing, then you have a better chance.
I'm sure a lot of you cats who have never had one were closer to getting one than you can probably imagine, but didn't because you didn't make a move. Don't be afraid to make a move on a girl if she's feeling you.
If you happen to get lucky and it works in your favor, don't fall for that "We have to be dating" bit. Right when she hits you with that, make yourself scarcely available and be forthright with your desire to not be in a relationship but still wanting to interact with her on that level. Say you want to date her, but you can't balance a full on relationship.
From there you should be able to handle biz.