I'm confused. 肉夹馍 (ròu jiā mó) isn't served in 饼 (bǐng). It is is served in 馍 (mó), hence the name. 馍 (mó) are never sweetened. Alternatively they are served in 白吉饼 (bái jí bǐng) which are also unsweetened and almost identical to 馍 (mó)
饼 (bǐng) are cakes, so usually sweetened. I would call them pancakes, sweetened or not. Certainly not bread..
This previous topic, dedicated to Rou Jia Mo, contains a recipe for 白吉饼 (bái jí bǐng).