Preheat oven to 350°F. Butter and flour a 9x5x2 1/2-inch metal loaf pan. Whisk first 4 ingredients in medium bowl to blend. Combine chocolate chips and walnuts in small bowl; add 1 tablespoon flour mixture and toss to coat.
Beat butter in large bowl until fluffy. Gradually add sugar, beating until well blended. Beat in eggs 1 at a time. Beat in mashed bananas, lemon juice and vanilla extract. Beat in flour mixture. Spoon 1/3 of batter into prepared pan. Sprinkle with half of nut mixture. Spoon 1/3 of batter over. Sprinkle with remaining nut mixture. Cover with remaining batter. Run knife through batter in zigzag pattern.
Bake bread until tester inserted into center comes out clean, about 1 hour 5 minutes. Turn out onto rack and cool.