Greetings.
I have experienced the same problem for quite some time. (I could go on at length about the stupidity of this design, not to mention the business of the Send mailbox turning into "Sending...", and a zillion other things in Mail, but I'll spare you.)
I've found this behavior to be pretty random. I go through stretches where Outbox is always there even when there's nothing in it, and other times when it's never there whether there's something in it or not.
Here's a suggestion for something that's mostly worked for me, but it requires a little prefatory explanation. I normally keep all my accounts offline. Another random weirdness of Mail that I've found is that when I'm offline and I send something, sometimes it stays in the Outbox (as I believe it should), but sometimes it sends right away even though I'm offline. I accidentally discovered that if there's something in my Outbox when Mail starts, that the Outbox seems to stay visible all the time after that, even when there's nothing in it (which is how I want it to be). So if I start Mail and Outbox isn't showing, I send myself a little dummy email and cross my fingers that it won't send right away, but stay in the Outbox. If it does send right away, I just have to keep trying until it doesn't (I think that going online and then back off may help with this). When I'm pretty sure that there's something in the Outbox (even though it's not visible and, as far as I can tell, the is NO WAY to verify this), I quit Mail and restart it. That has always made the Outbox show up, at least so far.
Good luck.