An interim review of Boeing's progress on building a fleet of new aerial tankers for the Air Force gives the company high marks for sticking to a strict budget. But government auditors are worried about some possible hurdles ahead. The first four test tankers have been built - they're essentially engineering test beds - and may be flown by the middle of next year.

Auditors of the Government Accountability Office saluted Boeing this week on holding down development costs, on acing the first design review and for holding firm on design parameters. But GAO officials also warned Boeing and the Air Force about challenges ahead - notably, a delay of between six months and a year on the start of initial operational testing and evaluation.

The oversight agency also pointed to problems cropping up in the tanker's software development. Auditors said that all testing may be in jeopardy if more re-testing is needed than expected. The audit report said that about 15.8 million lines of software code has been written and plugged into the testing program. The interim report also warned about manufacturing delays caused by late supplier deliveries, and the need to have both US and British airplanes available to be refueled in test flights. 

The new KC-46 aerial tankers are being built in Everett, Washington on the standard Boeing 7-67 airframe. The Air Force will call the tanker "Pegasus" after the winged horse of Greek mythology. Boeing is scheduled to build 18 new tankers by 2017, and eventually 179 total in the 51-billion dollar program.