Abstract: A storage apparatus for flatbed trailers includes a storage box which swings out to the side from beneath the trailer bed for loading and unloading. When in an up, traveling position, a lip on the front of the storage box engages a flange of the trailer side rail to support the front of the box, while a U-shaped frame connected to the trailer framework provides support beneath the rear of the box. Two arm members are pivotally mounted between the trailer framework and the sides of the box, and support the box while not in the up position. The pivotal attachments of the arm members to the box are located so that the box is unbalanced with the front biased downwards. Curved rail members, supported by the trailer framework, engage rollers attached to the rear of the box to control its upward movement. Dual-acting air cylinders are pivotally attached between the trailer framework and the arm members.