WASHINGTON -- Computer spies have broken into the Pentagon's $300 billion Joint Strike Fighter project -- the Defense Department's costliest weapons program ever -- according to current and former government officials familiar with the attacks.

Similar incidents have also breached the Air Force's air-traffic-control system in recent months, these people say. In the case of the fighter-jet program, the intruders were able to copy and siphon off several terabytes of data related to design and electronics systems, officials say,...