The youngest active stealth bomber in the U.S. turns 15 this year, and the other 19 B-2s in the Air Force fleet are nearly five years older. Meanwhile, the integrated defense systems they face have become much more sophisticated. Multi-static radar, which is now relatively common, is so sensitive that it can detect certain stealth craft. To stay ahead of such defense systems, the Air Force has budgeted $3.7 billion over the next five years to develop a successor to the B-2 that could be active by 2020. Actual designs of the new bomber are classified, but some secrets are already out.