Thermoplastic composites offer the potential for reducing the overall manufacturing costs of aircraft structures by allowing continuous production methods to be applied without the ancillary need for ovens or autoclaves by using in situ consolidation techniques. In the last 10 years, carbon-fiber/polyether-ether-ketone-based composites have become available with desirable combinations of strength, stiffness, and toughness properties. By combining the latest manufacturing techniques with these new materials and with new design methods, cheaper, lighter, and better-performing aircraft structures become a viable prospect. As such, a variable-stiffness unitized integrated-stiffener thermoplastic composite wingbox was developed, which was manufactured by laser-assisted automated tape placement with winding and in situ consolidation. The wingbox loads were determined by assuming its location to be at 85% of the wing semispan of a B737/A320-size aircraft. At this load, the wingbox was designed to buckle elastically. A full-scale structural test using a bespoke testing frame with representative bending moment and shear load was undertaken. Indeed, the wingbox buckled elastically at a load close to that predicted numerically. The current results highlight the potential advances that become possible in primary aerospace structures by combining fiber steering and in situ consolidation of carbon-fiber thermoplastic composites together with new blended, unitized structural concepts.