Boeing said on Thursday it hired former Northrop Grumman executive Colin Miller to head its Phantom Works research arm within the company's defense business unit.

Boeing has poached an executive from defense rival Northrop Grumman to run its Phantom Works division, which oversees research efforts for its Pentagon business and is involved in bidding for U.S. government projects.

After striking for almost two months, workers will return to factory floors in the Pacific Northwest, but must ensure safety procedures are followed as it begins building 737 jetliners.

Airline customers have grown increasingly upset over delays in getting new planes from Boeing — delays that started long before 33,000 machinists went on strike.