With the Pentagon expected to release the final bid request this week for the second round of bidding on a $40 billion contract to supply the US Air Force with refueling tankers, Boeing has threatened to withdraw from the contest unless the Department of Defense grants a six month extension to the bid process, so it can work on the design of a larger aircraft.   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 Sp