Boeing and Lockheed Martin have lodged a formal protest with the US Government Accountability Office (GAO) over the US Air Force's (USAF) next-generation long range strike bomber (LRS-B), contract award to Northrop Grumman.
The companies have jointly sent a 133-page brief in response to the USAF's filing.
Last month, Northrop defeated Lockheed and Boeing to win the contract to replace the USAF's ageing fleet of bombers.
Boeing said in a statement: "Boeing and Lockheed Martin team believe that the air force's selection process was irreparably flawed and therefore have decided to continue with their protest before the GAO."
According to the USAF, the independent estimate for the EMD phase is $21.4bn in 2010 dollars.
The new bombers will replace the USAF's ageing B-52 Stratofortress and B-2 Spirit fleets.
USAF spokesman major Robert Leese was quoted by DefenseNews as saying: "Although it is every competitor's right to file a protest, the air force is confident that the source selection team followed a deliberate, disciplined and impartial process to determine the best value for the war-fighter and taxpayer.
"The air force will fully support the GAO's independent process."
Currently, Northrop Grumman manufactures the B-2 bomber, while Boeing and Lockheed are working on developing the KC-46A tanker and F-35 Lightning II joint strike fighter (JSF) aircraft for the USAF under separate programmes.