Another observation to add to this and I strongly think this is how we'll advance.

The first set of missions that we see are "Trainee" missions. I observed earlier today that when my squadron returned, the mission they accomplished was grayed out while the incomplete missions were not. I'm going to put forth a theory that you have to complete all the missions in the "Trainee" to unlock whatever is after that and once you do that you'll unlock a brand new set of missions, from there once you complete enough sets you'll be able to get promoted to First Lieutenant.