Less than 20 minutes
That's how long it took for Brian Peddle to respond to my query about paper airplanes. Impressive.
The link he found: Paper Airplane History Page isn't definitive about whether the plane pre-dates the Wright Brothers, but it's pretty clear from all the data that it does. I'm happy to consider this case closed. Thanks.
PS Five minutes later, we find this link: Cecil Adams on paper airplanes. While I don't adore Cecil as I used to, he's in violent agreement with me. Brian has once again come through.