Preheat oven to 425 degrees. Using a rolling pin, flatten bread to about 1/8 inch thick; place on a baking sheet. In a small saucepan, melt butter over medium-low; lightly brush both sides of bread with half the melted butter (leave the rest in saucepan). Bake until lightly browned, about 4 minutes per side (leave oven on). Set aside.
Meanwhile, place saucepan with butter over medium heat. Add sugar; stir to combine, and bring to a boil (mixture will foam and bubble). Remove from heat. Stir in rum.
Place bananas on bread, overlapping slightly. Bake until warmed through, about 5 minutes. Remove from oven, and brush with rum mixture. Serve.