Preheat oven to 375°. Whisk together flour, sugar and milk just until blended; whisk in melted butter. Pour batter into a lightly greased 11x17 inch baking dish. Top with banana slices, and sprinkle with Streusel topping. Bake at 375° for 40 to 45 minutes or until brown and bubbly. Serve with ice cream.