WASHINGTON — NASA on Monday announced it has awarded nearly 270 million dollars to four companies, including Boeing and SpaceX, to help their pursuit of making a spacecraft to replace the US space shuttle.

The Houston, Texas-based aviation giant Boeing received the biggest contract -- $92.3 million -- as part of the second round of NASA commercial crew development program, or CCDev2.

Sierra Nevada Corporation, a Colorado-based company that is at work on its DreamChaser shuttle, won the second largest sum at $80 million.

SpaceX, the California-based company that last year completed its first successful test of an unmanned space capsule into orbit and back, won $75 million and Blue Origin of Kent, Washington received $22 million.

"These agreements are significant milestones in NASA's plans to take advantage of American ingenuity to get to low-Earth orbit, so we can concentrate our resources on deep space exploration," said NASA administrator Charles Bolden.

NASA awarded about $50 million in its first round of commercial crew contracts, CCDev1, which began in 2009.

The US space shuttle program is winding down later this year with final flights of Endeavour in April and Atlantis in June, ending an era of American spaceflight that began with the first space shuttle mission in 1981.

When the shuttle program ends, the United States hopes that private industry will be able to fill the gap by creating the next generation of spacecraft to transport astronauts into space.

Until then, the world's space agencies will have to rely on Russia's space capsules for transit to the orbiting International Space Station.