Cost and scheduling challenges have pushed the US Air Force’s B-52 Stratofortress modernization program billions over budget, according to a new report.

The aging B-52 has long been a workhorse of the Air Force, which is replacing the bomber’s decades-old engines to keep the aircraft flying into the 2050s. But the upgrades have run into delays and rising costs.

Last week, the Government Accountability Office, a government watchdog agency, released a report on the current status of the B-52 modernization and sustainment efforts. Of the 13 upgrades that the bombers are getting, 10 are experiencing cost, schedule, or performance challenges.

The mos…