In a large bowl combine 3 cups flour, the salt, and yeast. Add the warm water. Stir until flour mixture is moistened (dough will be very sticky and soft). Cover; let stand at room temperature for 4 to 24 hours.
Generously sprinkle additional flour (3 to 4 tablespoons) on a large piece of parchment paper. Turn dough out onto floured paper. Sprinkle top of dough mixture lightly with additional flour (1 to 2 tablespoons); using a large spatula, gently fold dough over onto itself. Sprinkle lightly with additional flour (1 to 2 tablespoons). Cover; let rest for 15 minutes.
Grease a 5- to 6-quart Dutch oven or heavy pot with a diameter of 8-1/2 to 9-1/2 inches; sprinkle cornmeal over bottom and about 2 inches up the sides. Gently turn dough into prepared Dutch oven, using a spatula to help scrape dough off the paper (some dough may remain on the paper). Cover; let rise at room temperature until dough has risen by about 1 inch in the pan (1 to 2 hours).
Preheat oven to 450 degrees F. Cover Dutch oven with a lid or foil; bake for 30 minutes. Uncover; bake 10 to 15 minutes more or until top is golden brown. Immediately remove bread from pan. Cool completely on wire rack.