A rotor blade configured to be installed on a rotary wing aircraft includes a spar including an inboard portion extending from a root of the rotor blade to a first spanwise position of the rotor blade and an outboard portion meeting the inboard portion at the first spanwise position. The rotor blade further includes an upper skin and a lower skin coupled the outboard portion of the spar. The first spanwise position is defined by a most inboard point at which the upper and lower skins are coupled to the spar. The inboard portion includes a top layer, a bottom layer, and an inner cavity. The top layer has a substantially constant first thickness and the bottom layer has a substantially constant second thickness, and the distance between an outer surface of the top layer and an outer surface of the bottom layer is substantially constant.