Ask faculty who know you well and can speak at length regarding your abilities and interests.
Only ask academic faculty���your supervisor at the bank may know you well but is not in a place to discuss your academic qualifications.
It���s okay to include a letter from a non-historian���many good applicants have double majors and minors and faculty in those programs may be able to speak about your abilities better than a random History professor with whom you have taken one class.
Give faculty at least a month���s lead time to write that letter. You do NOT want to be that student who requests a letter in 72 hours. Faculty are not likely to be effusive in their praise for a student who is disrespectful of their time in that way.