Uncured composite materials (prepreg) are provided that are stable at room temperature over relatively long periods of time. The prepreg includes a solid epoxy resin matrix composed of a crystalline epoxy resin and a solid curing agent. The curing agent is preferably crystalline. The prepreg may include one or more property enhancement layers that increase the tackiness of the prepreg, increase the flexibility of the prepreg and/or reduce the amount of solid resin matrix that flakes from the prepreg during handling.