A forming box for use in dry forming of a mat of fibrous material, and a forming method. The forming box comprises a housing with an open bottom providing direct access for fibers onto an underlying forming wire and a vacuum box underneath said forming wire; an inlet for supplying fiber material into the inside of the housing; and a number of fiber separating rollers in one or more rows in the housing between the fiber inlet and the housing bottom. An endless belt screen is provided inside the housing having an upper run between two rows of spike rollers and a lower run closer to the open bottom of the forming box. Fiber clumps captured on the lower run of the belt screen are conveyed above fiber separating rollers and are released from the upper run of the belt screen into contact with the fiber separating rollers.