When Paul G. Kaminski completed his graduate work in 1971 with degrees from Massachusetts Institute of Technology and Stanford, he started building advanced airplanes for the Air Force. By the time he stopped several decades later, he had played a pivotal role in producing a flock of new weapons, including radar-evading stealth aircraft.

If Mr. Kaminski were coming out of school today, chances are he would be going to work for the likes of Microsoft or Google.

Over the last decade, even as spending on new defense projects has reached its highest level since the Reagan years, the Pentagon has increasingly been losing the people most skilled at managing them. That brain drain, defense experts like Mr. Kaminski say, is a big factor in a breakdown in engineering management that has made huge cost overruns and long delays the maddening norm.

Mr. Kaminski’s generation of engineers, which was responsible for many of the most successful defense projects of the 1970’s and 80’s, is aging. But declining numbers of the nation’s top young engineers, software developers and mathematicians are replacing them. Instead, they are joining high-tech companies and other civilian firms that provide not just better pay than the military or its contractors, but also greater cachet  what one former defense-industry engineer called “geek credit.”