"...will gradually replace aging B-1 Lancer and B-2 Spirit b

https://www.af.mil/News/Article-Display/Article/3235250/b-21-raider-makes-public-debut-will-become-backbone-of-air-forces-bomber-fleet/#:~:text=The%20B%2D21%20is%20designed,Spirit%20bombers%20now%20in%20service.

 The B-21 is designed to be a more capable and adaptable, state-of-the-art aircraft that will gradually replace aging B-1 Lancer and B-2 Spirit bombers now in service.