The I-301 airframe was partially made of "delta wood": a material composed of very thin (0.35–0.55 mm) layers of birch or pine wood veneer, and a phenol-formaldehyde resin known as VIAM-B-3, which together were baked at high temperatures and pressures. Delta wood was used for critical parts of the airframe. This novel construction material had tensile strength comparable to that of non-hardened aluminum alloys and only 30% lower than that of precipitation hardened D-1A grade duralumin. It was also incombustible and completely invulnerable to rot, with service life measured in decades in adverse conditions. During production of the prototype, it was discovered that the adhesive used in delta wood caused skin irritation and safety procedures needed to be devised for workers.