WASHINGTON (Reuters) - Computer spies have repeatedly breached the Pentagon's costliest weapons program, the $300 billion Joint Strike Fighter project, The Wall Street Journal reported on Tuesday.The newspaper quoted current and former government officials familiar with the matter as saying the intruders were able to copy and siphon data related to design and electronics systems, making it potentially easier to defend against the plane.The spies could not access the most sensitive material, which is kept on computers that are not connected to the Internet, the paper added.Citing people briefed on the matter, it said the intruders entered through vulnerabilities in the networks of two or three of the contractors involved in building the fighter jet.