By Andrea Shalal-Esa

WASHINGTON (Reuters) - Lockheed Martin Corp (LMT.N: Quote, Profile, Research) has spent a significant amount to make its bid for a $15 billion contract for a fleet of combat search and rescue helicopters more attractive by making it less risky, a top official told Reuters this week.

Industry bids are due on May 22 for the Air Force project, which was initially awarded in 2006 to Boeing Co (BA.N: Quote, Profile, Research) for a variant of its twin-rotor Chinook helicopter.

But the Air Force was forced to redo the competition after the Government Accountability Office upheld protests filed by losing bidders Lockheed and United Technologies Corp (UTX.N: Quote, Profile, Research).

Lockheed plans to offer a variant of the EH101 Merlin helicopter built by AgustaWestland, a unit of Italy's Finmeccanica (SIFI.MI: Quote, Profile, Research). It is the same aircraft that won the U.S. Navy's VH-71 presidential helicopter competition in 2005.

Schedule delays and cost overruns on that program resulted in Lockheed getting a low rating for past performance during the initial competition for the Combat Search and Rescue helicopter. Lockheed and the U.S. Navy say the program has been restructured and is doing better.

"We're giving the Air Force the absolute lowest risk proposal," Lockheed vice president Dan Spoor told Reuters in an interview. He said Lockheed was confident that its moves would make the bid more attractive to the Air Force.

The company has focused on improving the past performance marks and eliminating a "high risk" rating it got the first time for new rotor blades that had not been tested.

Spoor declined to estimate how much Lockheed had spent on the effort, saying only that the amount was significant.