Only the final two Northrop B-2s to be delivered will be at the definitive Block 30 standard featuring a 'mature low observable standard'. Of the planned total purchase of 20 stealth bombers, the first 18 will be modified to the definitive Block 30 standard during programmed depot maintenance at the end of the decade.EarlyB-2 aircraft delivered will be at an interim Block 10 standard with the ability to deliver nuclear and conventional dumb bombs and an 'initial low observable signature', while intermediate Block 20 aircraft will feature a precision guided weapon capability. Testing of the B-2 during 1991 revealeda worse than expected signature, while recently, there has been speculation that the USAF has Eased its signature specifications to assist the aircraft in meeting requirements.