Airbus is a threat, but they have a huge obstacle Boeing doesn't: they don't build planes in the US except for the A319-320-321. Any defense contract won with an Airbus airframe as its basis (e.g. NG's KC-X submission based on the Airbus A330 MRTT) also includes the expense and possible difficulty building a factory in the US to manufacture them. Boeing may have a bad reputation for screwing up manufacturing, but that's a known quantity vs. the giant question mark that is the quality of a theoretical Airbus factory--- run by a US defense contractor rather than Airbus--- that hasn't been built.