A dump body for a truck has rear left and right pillars, a tailgate pivotally connected to the pillars, left and right upper rails connected to the rear left and right pillars at their rear ends, a front wall connected to the front ends of the upper rails, a left side wall connected to the front wall, the left upper rail and the rear left pillar, a right side wall connected to the front wall, the right upper rail and the rear right pillar, a floor connected to the front wall, the rear pillars, and the lower ends of the side walls, and front and rear support assemblies connected to an underside of front and rear portions of the floor. A majority of an underside portion of the floor located longitudinally between the support assemblies is free of structural support members. A truck having the dump body is also described.