The Northrop Grumman B-2 Spirit combines the high aerodynamic efficiency of the "flying wing" design with stealth technologies, producing a bomber invisible to even sophisticated air defense radar. Northrop Grumman built this example without engines or instruments. This airframe was stressed to the breaking point to document the strength of the design. Engineers calculated that the structure would withstand 150% of the design specification. The wing finally cracked at 161%! Visitors looking carefully can see a repair underneath the wing.