The US government has decided to reopen bidding for a $40 billion contract to supply the US Air Force with refueling tankers, after Boeing complained about the original award of the contract to Northrop Grumman and its European partner.   The contract to build 179 refueling tankers over 15 years was awarded on February 29 to Los Angeles-based Northrop Grumman and its partner, EADS North America, a subsidiary of European Aeronautic Defence and Space Co., the parent of Airbus.   The decision has proved extremely controversial, however.