Combine first 5 ingredients in a large bowl; stir well. Set aside.
Beat butter at medium speed of an electric mixer until creamy; gradually add sugar, beating well. Add eggs, one at a time, beating after each addition. Add banana and milk to creamed mixture; add to dry ingredients, stirring just until dry ingredients are moistened. Stir in walnuts.
Pour batter into a greased and floured 9- x 5- x 3-inch loafpan. Bake at 350° for 1 hour or until a wooden pick inserted in center comes out clean. Let cool in pan 10 minutes; remove from pan, and let cool on a wire rack.