A rotor blade (1) provided with an outer covering (2) and with at least one load take-up spar (10) associated with at least one pin-receiving bushing (3). Each spar (10) is a compartmentalized spar comprising centrifugal force take-up means (20) incorporated within a twisting stress take-up casing (30), said centrifugal force take-up means (20) comprising at least two boxes (21, 22), each having said pin-receiving bushing (3) of the spar (10) passing therethrough, each box (21, 22) including a closed retention belt (23) extending in the spanwise direction of the blade, the retention belt (23) of one box (22) surrounding the retention belt (23) of another box (21), each retention belt (23) being provided with unidirectional fibers (24) wound around said pin-receiving bushing (3), said casing (30) being provided with inclined fibers (31) presenting an angle relative to said unidirectional fibers (24).