I was a RM in a US bank before leaving corporate banking about 2 years ago.

Joined as a management associate, did my rotations and finally decided to get "placed" in the corporate bank.

For my team, quite a number of the RMs were actually from operations before and being from operations does have advantages:
1. you would have worked directly with the RMs
2. you know the bank processess well
3. you would have accumulated certain product knowledge.

We do also take in fresh graduates and usually take from the local universities. A good number of them were actually interns who had interned in our team or in other teams. They however start off as RAs (relationship associates) before gradually moving on to become RMs.