More female executives are rising to the top levels of the largest defense companies, setting a standard that industry officials say may help encourage more women to enter the field. This week, Phebe Novakovic is set to become president and chief operating officer at Virginia-based General Dynamics after serving as executive vice president of the company’s marine systems group. The move puts Novakovic in line to replace Jay Johnson, General Dynamics’s chairman and chief executive, though the company has not made any formal decision. “That’s the start of a very disciplined succession rhythm here,’’ Johnson said of Novakovic’s new role during a call with investors last week. “We’ll continue to work very closely together.’’