The U.S. Air Force contracted Boeing to produce GBU-57 Massive Ordnance Penetrator precision-guided “bunker buster” bombs, a U.S. Department of Defense press release said.

“The Boeing Co., St. Louis, Missouri, has been awarded a $20,970,424 contract for the procurement of GBU-57 massive ordnance penetrators,” the Thursday, February 8 release said.

The contract is a sole-source acquisition, and work is expected to be complete by July 31, 2020.

The release did not specify the number of bombs to be procured, but in 2011 the U.S. Air Force ordered eight of the weapons plus supporting equipment for $28 million.

The GBU-57 Massive Ordnance Penetrator (MOP) is a 30,000-pound (14,000 kg) class GPS-guided “bunker buster” bomb employed only by the B-2 Spirit stealth bomber. The 20-feet (6m) long weapon designed to “attack deeply-buried and hardened bunkers and tunnels” carries 5,300 pounds of explosives and is said by the U.S. Air Force to be able to penetrate up to 200 feet (60m) of concrete before exploding.