How to Make It
Combine 1 cup warm water and yeast; let stand 5 minutes. Stir in banana.
Lightly spoon flours into dry measuring cups; level with a knife. Add 2 cups bread flour, wheat flour, soy flour, and salt to yeast mixture; stir until a soft dough forms. Turn dough out onto a floured surface. Knead until smooth and elastic (about 8 minutes); add enough of remaining 1/4 cup bread flour, 1 tablespoon at a time, to prevent dough from sticking to hands (dough will feel sticky).
Place dough in a large bowl coated with cooking spray; turn to coat top. Cover; let rise in a warm place (85°), free from drafts, 45 minutes or until doubled in size. (Press two fingers into dough. If indentation remains, the dough has risen enough.) Punch dough down; cover and let rest 5 minutes. Roll dough into a 14 x 7–inch rectangle on a floured surface. Roll up tightly, starting with a long edge, pressing firmly to eliminate air pockets; pinch seam and ends to seal. Place roll, seam side down, in a 9 x 5–inch loaf pan coated with cooking spray. Coat top with cooking spray. Cover; let rise in a warm place 30 minutes or until doubled in size. (Press two fingers into dough. If indentation remains, dough has risen enough.)
Preheat oven to 400°.
Bake at 400° for 1 hour or until lightly browned on bottom and sounds hollow when tapped. Remove from pan; brush with butter. Cool completely.