USAF buys Airbus tankers?

This is probably old news but…. Apparently the USAF is buying 179 tankers from EADS/Airbus/Northrop-Grumman to the tune of $35 billion.

http://www.msnbc.msn.com/id/23413217/

How could this happen? Aren’t US makers like Boeing the default choice for these contracts?