United did in fact fly IAD
with a B767-300ER but axed the route in 1994 or 1995. Apparently, the loads were not that good in the premium cabins (not much business travel between Washington DC and Zurich) and so the route was better served through the LH
Even Swiss had trouble making Washington-Zurich work, and it too has axed the route.
Current US airlines with service to Zurich are:
Continental (Daily from EWR
Delta (Daily from ATL
American (Daily from JFK