First composite air frame was the less-than-successful Beech (Raytheon) Starship. FAA certification was held up not for structural reasons, but because the fly-by-wire avionics weren’t shielded from lightning induced EMI by a metal airframe. A lot of the weight and fuel savings evaporated when they had to be retrofitted with shielded wiring.
I'm sure Boeing got a heads up from their experience.