Apply to a range of programs simply because that's the smart thing to do regardless of your background. I don't know how important the name of the field on your degree is because it seems to vary by program. I got my BA in English literature and my MA in anthropology. During this past admissions season, a top 25 program rejected me for the PhD program (but accepted me into their MA program) because they require a History MA, and a top 10 program accepted me into their first string. I've never actually taken a formal history course at the undergraduate or graduate level. However, my research in other fields has always been historical in approach. (In my statement of purpose, I explained how my experience writing in other fields led me to the conclusion that I was more a historian than something else, and thus I needed formal training in a history program. You should probably address this issue in your statement of purpose too.)
I don't know if my experience is abnormal, but if your statement of purpose intelligently explains why you've come to history at this point in your academic career and your writing sample proves that you're capable of insightful research of a historical nature, you probably have as good a shot as any.
Gaijinhakuryu got a reaction from Safferz in A Tactical Conundrum