To prepare sponge, dissolve yeast in warm water in a large bowl; let stand 5 minutes. Lightly spoon 1/2 cup rye flour and 1/4 cup bread flour into dry measuring cups; level with a knife. Add 1/2 cup rye flour and 1/4 cup bread flour to yeast mixture, stirring with a whisk. Cover and let stand in a warm place (85°), free from drafts, 2 hours.
To prepare bread, lightly spoon 2 cups bread flour and 1/2 cup rye flour into dry measuring cups; level with a knife. Add 2 cups bread flour, 1/2 cup rye flour, 1/2 cup water, caraway seeds, and salt to sponge; beat with a mixer at medium speed until smooth.
Turn dough out onto a lightly floured surface. Knead until smooth and elastic (about 10 minutes); add 2 tablespoons bread flour, 1 tablespoon at a time, to prevent dough from sticking to hands.
Shape dough into a round loaf; place loaf on a baking sheet coated with cooking spray. Cover and let rise 1 1/2 hours or until dough has doubled in size. (Press two fingers into dough. If indentation remains, the dough has risen enough.)
Preheat oven to 425°.
Uncover and pierce loaf 1 inch deep in several places with a wooden pick. (Stencil an initial on bread, if monogramming.) Bake at 425° for 30 minutes or until loaf is browned on bottom and sounds hollow when tapped. Let stand 20 minutes before slicing.