NROTC to medical school is a viable option. I had many classmates at USUHS who were former ROTC. I'm not as familiar with the process as I am for the Academies; however, sometime during your later years in college you would apply for the medical corps with the blessing of your Commander and then your package would be routed up the chain for approval. I think those in ROTC have a higher percentage of people allowed to go Medical than USNA; however, don't quote me on that.
As an aside, even if you are interested in going to medical school DO NOT, I repeat DO NOT, major Pre-med. Find something that you are interested in to major and add on whatever pre-requisites for medical school are not in the major you chose. Being a pre-med will give you no leg up in medical school and the admissions committees don't care what your major is and get tired of seeing the same ole Biology, Chemistry, Pre-med majors. If biology or chemistry interest you then do it, otherwise go with something else. Even if it's not a science major you are still fine.
Don't forget you don't have to go ROTC to do the HPSP program or USUHS which are both ways to get medical school paid for.