Investing.com -- A $21 billion U.S. Air Force program managed by Boeing Co. to modernize the B-52 bomber fleet is facing cost increases and schedule delays during its early development phase, according to a Government Accountability Office report released Friday.

Ten of 13 upgrade projects for the aircraft are experiencing cost, schedule or performance challenges, the GAO said. The report highlighted a Rolls-Royce Holdings Plc engine upgrade and a radar project as facing all three types of problems.

The Air Force operates 76 B-52 bombers from the original 744 produced. The aircraft first entered service in 1955 and has significantly outlived its original lifespan, the GAO noted. The upgrade program aims to keep the aircraft operational through the 2050s alongside B-2 and B-21 stealth bombers.

The B-52 can fly 8,800 miles without refueling, conduct conventional and nuclear bombing missions, and carry 70,000 pounds of ordnance. The bomber has been flying Iran missions since March.

The F130-200 engine program represents the costliest element of the upgrade. The Air Force's cost estimate for the engine development phase has increased to about $15 billion from $11 billion in 2018. The initial operational capability has been delayed to 2033, representing a five-year delay from the original target and an additional 15-month delay since 2023.

The GAO warned the engine program will likely face challenges as the Air Force plans to begin production later this decade with little or no developmental flight testing. The report said this approach could lead to more delays and cost overruns as problems are discovered.

The cost increase prompted the Air Force to initiate an affordability study that was ongoing as of March, according to the GAO.

The engine program faces risks from concurrency, which is the overlap between development testing and starting production, the GAO said. This practice often leads to cost and schedule growth when testing reveals flaws requiring design changes.

Air Force officials told the GAO they recognize the risks but are willing to accept them to avoid additional schedule delays.

Once production begins, the service plans to order between 600 and 650 engines from Rolls-Royce and order aircraft subsystems from Boeing.