Boeing Co. said on Monday the head of its big defense division, Leanne Caret, will retire after nearly 35 years with the aviation company. Caret, who is 55 years old, will be succeeded by Ted Colbert effective April 1. Caret oversaw development of the successful MQ-25 drone program and the U.S. Air Force’s new training jet. She was also in charge of the KC-46 midair tanker, which has been plagued by numerous problems and cost overruns. Boeing’s defense unit, which has the U.S. government as one of its biggest customers, generated revenue of $26.5 billion in 2021, overtaking its commercial airplane business. Colbert, who now heads Boeing Global Services, will become president and chief executive officer of Boeing’s Defense, Space & Security business. The global services unit handles jet maintenance, repair and overhaul, data analytics and pilot training. Caret, who had taken over the defense and space role in 2016, …