Every business is a bit different, but we bootstrapped several very successful companies in the B2B space and here are the milestones we hit. Some were planned goals, others I can see clearly in retrospect. I will say that as a bootstrapper, the single most significant milestone is positive cash flow. It was our goal from day one and almost everything we did at the start was driven by that goal

1 Beta test
2 Product launch
3 Users (hopefully paid but not always)
4 Paid users
5 First employee
6 positive customer feedback
7 Marquis customer (for us Apple and Harley Davidson)
8 Significant volume or dollar goals (ie first 100 customers, first month gross over $100,000. First month profitable, first $1,000,000 etc).
9 First time Vc's or private equity guys call to see if you need money.
10 If it is what you are planning, successful exit at a high multiple.

There were many more goals that we set along the way that were very specific to our business, but it was critical that we set them and shared them very visibly for the most part with the company. We also made sure when we hit one that we celebrates the success.

Hope this helps, would be happy to do a brief call to discuss your business specifically and talk about any goals you should set. Good luck it is a fun (albeit sometimes hard) ride.