A stiffened composite panel is fabricated using modular tooling. Composite pre-preg is laid up and formed over forming block modules each of which is assembled by co-linearly arranging a plurality of forming block segments. The formed stiffeners are respectively transferred to individual cure tool modules that are assembled by co-linearly arranging a plurality of cure tool module segments. The stiffeners are assembled by arranging the cure tool modules side-by-side, and a composite skin is placed on the assembled stiffeners. The panel is vacuum bagged using a segmented vacuum bag.