Sub-structures of aircraft structures mainly consist of stiffened shells such as fuselage frames, ribs and multi-cell box beams. Conventionally, these stiffened shells are manufactured through a process wherein shells and stiffeners are fabricated separately and then are integrated either through mechanical fastening and adhesive bonding. Co-curing is an integral molding technique that can greatly reduce the part count and the final assembly costs for composite materials. This article presents a simulation of integral manufacturing of a three-cell composite box beam by vacuum assisted resin infusion process. To validate the model, the characterization tests of both resin and reinforcement materials were carried out. Porosity and permeability testing of the reinforcement materials were conducted. Moreover, the effect of stacking sequence and vacuum level on the preform porosity were investigated. Additionally, the resin viscosity measurements were performed and the influence of temperature and curing on resin viscosity were examined. Having obtained the characterization data, vacuum infusion model was validated using RTMWorx software and then simulation of a three-cell composite box beam was conducted.