Heat oven to 350°F. Spray bottom only of 8x4-inch or 9x5-inch loaf pan with cooking spray. In medium bowl, mix flour, baking powder, baking soda and salt. In large bowl, beat granulated sugar, oil, egg whites, bananas, 1/4 cup milk and 1 teaspoon vanilla until well blended. Stir in flour mixture until blended. Stir in walnuts; spread in pan.