This is not to say that the Airbus is out of the competition. There are supposed to be three tanker competitions, KC-X, KC-Y, and KC-Z. As I have said before that the aircraft proposed by Airbus is more suited to the KC-Y competition that to the KC-X and I think would make a great choice for that competition. The solution? buy the Boeing KC-767 for the KC-X variant, the Airbus for the KC-Y variant, and wait and see if the 747 or the A380 will be chosen for the KC-Z variant.
As I've said elsewhere, I think the USAF would be far more prudent combining all the competitions and procuring one type, with the reduced costs of having just a single support infrastructure etc. As for Airbus putting the A330 forward for KC-X; they did really have an option. One could argue the original requirement was written to favour a certain Seattle based company...
...whcih may then have led to certain elements in the USAF realising the Aibus product compared rather favourably.

The 767 was pushed at the USAF back in 2001 when they were on the verge of shutting down the line and shifting work to its replacement the 787. And like C said, the requirements were written in favor of Boeing, and Congress back then even questioned it "why would you lease/buy a new plane that is only slightly better then the plane it was replacing?" but the USAF never addressed the question, and just said "this is what we need and want", hence the every one always saying "the original plane was supposed to be smaller, not larger!". And also Airbus tried back then by them selves to compete with the 767 with the A310, but by comparison the 767 was far better, and that's why it was chosen back then.
Something I was thinking about while watching a KC-135 take off, wing pods, Boeing has been having a hard time getting those working right, and the air flow issue solved on the Italian Tankers, and these are supposed to be the same one used on the US version, but they are behind on the development of these, not something that you want to hear when ordering 179 new planes that are all supposed to be fully capable from the start.
But as far as what C said about just getting one plane, that comes back to the saying "putting all your eggs in one basket", it's better to have 2 options then to rely on one plane to try and do it all. Yeah it is cheaper up front and saves you money right off, but after a while you will want options to do more, and now you have to pay to modifie your plane to get it to do what you want. And it is because of this reason why the USAF has not listened to Congress and replace all C-5's with C-17's, a move like that would limited what you could do.
But then with politics playing a major role, they may just throw that out the window and say "this is what you need, and this is what you'll get!" As they have done in the past with fighters and some other things.
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Boeing better use a special 767 with higher take off weight, center tanks for more fuel and more powerful motors, or just do what every one thinks they should have done in the first place and make a KC-787, and not a paper plane like the USAF's Paper tanker that was never built.
Northrup, on the other hand can now use the A330 Freighter, with it's stronger frame, and build center tanks so it has a higher take off and off-load fuel ability.
This would make for a true Medium size tanker face off, with a fly off.