The present invention refers to the configuration and manufacturing process of a rib (22) for the construction of an aircraft torsion box. In the method a flat stack of plies of composite material (13) is layered up, which is then cut to form a flat pre-form (14) having an outer contour (15) configuring flanges, and an internal contour configuring two or more diagonal truss (16) with flanges (17) at opposite sides. The flat pre-form (14) is press-formed to fold the flanges of the outer and internal contour to form a rib pre-form, which is finally cured. The invention also refers to a composite rib (22) obtained as an unitary body by conforming a single pre-form of stacked plies. The invention allows the manufacture of the rib (22) in one-shot process, integrating all its components, such as the assembly time and cost of the rib are minimized.