UPDATE 1-US Air Force awards $13 bln Doomsday plane contract to Sierra Nevada

(Adds details on contract in paragraphs 3, 6 & 7)

April 26 (Reuters) - The U.S. Air Force said on Friday that it has awarded a $13 billion contract to Sierra Nevada Corp to develop a successor to the E-4B, known as the Doomsday plane due to its ability to survive a nuclear war.

The Survivable Airborne Operations Center (SAOC) project is intended to replace the aging 1970s-era aircraft, which is approaching end-of-service life, an Air Force spokesperson said in a statement.

Work on the SAOC will be carried out in Colorado, Nevada and Ohio and is expected to be completed in 2036, the Air Force added.

To satisfy operational requirements, the weapon system will comprise of a commercial derivative jet hardened and modified to meet military requirements.

Reuters in December reported that the U.S. Air Force eliminated Boeing from its competition to develop a successor to the E-4B Nightwatch.

While typically used to transport the U.S. secretary of defense, the E-4B is designed as a mobile command post capable of withstanding nuclear blasts and electromagnetic effects, allowing U.S. leaders to deliver orders to the military in the event of a national emergency.

The E-4B is also capable of refueling mid-air and features conference and briefing rooms and advanced communications equipment.

The Air Force currently operates four E-4B aircraft with at least one on alert at all times. The fleet of highly-modified Boeing 747-200 jumbo jets has become increasingly difficult and expensive to maintain as parts become obsolete.

The E-4B is expected to reach the end of its service life in the early 2030s. (Reporting by Nathan Gomes in Bengaluru; Editing by Shailesh Kuber)