It was designed jointly by McDonnel Douglas and Northrop and is an evolution of the earlier Northrop YF-17 which competed against the F-16 in the Lightweight Fighter Program.

The YF-17, in turn, was actually an evolution of the even earlier Northrop F-5 Freedom Fighter.

Not trying to be a pedant, honestly. The F/A-18 just has a fairly interesting design heritage, and that heritage is pretty much entirely outside of Boeing. They're just the ones that ended up with the production after industry consolidation.