In medium bowl, mix flour, sugar, orange peel, baking powder and salt. In small bowl, with wire whip or fork, beat milk, peanut butter, butter and egg until well blended. Stir peanut butter mixture into flour mixture just until flour is moistened. Pour batter into greased 9 X 5" loaf pan. Bake at 375 degrees for 1 hour or until toothpick inserted in center comes out clean. Cool in pan on wire rack 10 minutes. Remove from pan and cool completely on rack before slicing or storing.