not that long - 10ish years. 15 by the time it enters service. sounds like mostly new guts.. they didn't have to reinvent the stealth part.

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https://en.wikipedia.org/wiki/Northrop_Grumman_B-21_Raider

As of 2022, the B-21 is expected to enter service by 2026 or 2027.[4] It is to complement and eventually replace the Air Force's Rockwell B-1 Lancer, Northrop Grumman B-2 Spirit,[5][6] and Boeing B-52 Stratofortress bombers.[7][8][9]