WASHINGTON — Lockheed Martin Corp., the world’s largest defense company, said Wednesday that about 25 percent of its executives opted for a voluntary retirement program designed to cut costs as defense spending slows.

More than 600 vice presidents and directors applied for the program offered in July, Bethesda, Md.-based Lockheed said in a statement Wednesday. Executives taking the retirement plan have begun leaving, and all will have gone by February, Jeffery Adams, a Lockheed spokesman, said in a telephone interview.