Preheat oven to 350 and grease a 9x5x3 inch loaf pan. Mix dry ingredients for the bread. Make a well in the center.
Mix mashed bananas, sugar, egg and melted butter. Pour into the dry ingredients and stir until just moistened.
Pour the batter into the greased loaf pan.
Combine brown sugar and flower. Cut in butter with a pastry blender (or use your hands–that is a good way to make sure the butter gets thoroughly mixed through).
Pour the crumb topping on top of the batter and bake the bread for 55-60 minutes. (For the last 15-20 minutes, I covered it with foil to keep the topping from burning.)