Preheat oven to 350°F. Coat a large loaf pan with cooking spray.
Peel and mash 3 bananas. Peel and cut 2 bananas into small chunks. Set both aside.
In a large bowl, sift together both types of flour, baking powder, baking soda and salt; set aside.
In another large bowl, using an electric mixer, combine butter and sugar until light and fluffy. Add egg; beat to incorporate.
Alternating in batches, add mashed bananas, flour mixture and milk to bowl with butter mixture. Combine well but do not over beat; fold in walnuts and banana chunks.
Turn batter into prepared pan; bake until a tester inserted in center of bread comes out clean, about 1 hour. Cool in pan for 15 minutes and then turn out onto a wire rack to cool more. Cut into 18 slices.