An aircraft wing box is disclosed having a fuel tank with a fuel-tight boundary, upper and lower covers, forward and aft spars, and a partition including an inboard portion, an outboard portion, and a third portion between the inboard and outboard portions. Each cover is attached to each spar, the inboard portion of the partition is joined to each cover and joined to one of the spars, the outboard portion of the partition is joined to each cover and joined to one of the spars, each cover is joined to the partition. The inboard part, outboard part and third part of the partition are integrally formed as a single-piece; and the single-piece provides part of the fuel-tight boundary of the fuel tank.