Prepreg composite materials that include a fiber layer and a prepreg resin composed of a bismaleimide resin component, a thermoplastic toughening agent and a resin distribution stabilizer. The bismaleimide component is an amorphous mixture of at least three different bismaleimide monomers and a co-curing agent. The prepreg is useful in panel construction where a honeycomb core is sandwiched between two sheets of composite material.