13 So the king sent a third captain with his fifty men. This third captain went up and fell on his knees before Elijah. “Man of God,” he begged, “please have respect for my life and the lives of these fifty men, your servants! 14 See, fire has fallen from heaven and consumed the first two captains and all their men. But now have respect for my life!”
So, no matter whether “captains” obtain their rank by inheritance or skill, they are commanders of a troop of 50 much like a modern military captain would be. They are above squad and platoon commanders but below generals. I doubt seriously that an ancient Israelite army would have been organized in the same detail a modern one is – probably no majors or colonels and so on, but they probably did have unit sizes that could be moved around a battlefield in large numbers and those would have been headed by “captains”.