A gatebox system for delivering fire retardant fluid from a firefighting aircraft. A first and second gate opening are formed through a lower portion of a gatebox. The gates are hingedly connected along edges of the openings. One or two drive shafts in the gatebox are rotatable in gates-closing and gates-opening directions. Crank arms are fixed to the drive shafts and coupled to the gates by connecting links, which define an over-center geometry while the gates are at a closed position, such that weight of the fluid on the first gate induces torque on the drive shaft in the gates-closing direction. Rotation of the drive shafts in the gates-opening direction enables control of the fluid flow from the hopper according to an angular position of the drive shaft.