Processes for fabricating an integrated fiber-reinforced cured resin-composite panel structure include forming a panel preform assembly by (i) positioning pre-cured fiber-reinforced resin-composite tubular stiffeners onto an uncured base skin, and (ii) applying uncured fiber-reinforced resin-composite overlapping layers onto the pre-cured stiffeners so that at least lateral edges of the overlapping layers are laminated to a corresponding region of the base sheet; and thereafter curing the fiber-reinforced resin-composite base skin and overlapping layers to thereby form an integrated composite panel structure.