Stir all-purpose flour, whole wheat flour, brown sugar, instant yeast, and salt together in the bowl of an electric mixer fitted with a dough hook attachment. Whisk water and oil together in a cup; pour into flour mixture. Mix on low speed until dough pulls away from the sides of the bowl into a ball.
Turn dough out onto a lightly floured surface; knead just until dough is smooth, about 1 minute. Divide and shape dough into 3 balls for small loaves or 2 balls for larger loaves. Dust loaves with flour and place onto a baking sheet; cover with a towel, and let rise until doubled in size, about 25 minutes.
Preheat oven to 375 degrees F (190 degrees C). Slash an "X" about 1/4-inch deep in tops of loaves with a sharp knife.
Bake in the preheated oven until loaves are golden brown and sound hollow when tapped, about 25 minutes. Remove from oven to cool completely on a wire rack.