In recent times, airplanes can take 20 or so years to go from design to combat-ready. The design of the B-21, so far only seen in concept art, resembles the Air Force's last new bomber, the B-2 Spirit. The Spirit felt futuristic when it debuted in the early 1990s, and much of its design is still cutting-edge in 2016. With the B-21, there's not much of the stealthy flying wing that needs reinventing, and existing systems can be adapted to the plane, rather than built from scratch. That means the Air Force expects the plane to be ready in 10 years. That's a long time for most technology, but fast by modern warplane design standards.