The Headway MLC (Maximum Legal Carry-on) from Patagonia is a very functional gear bag that works better as a duffel due to it’s uncomfortable (but hideable!) backpack straps.
Other Bags to Check Out:
The Litehaul 38 from Kathmandu is capable, stylish, boasts a very interesting pocket configuration and a more comfortable than normal harness system.
The Setout from Tortuga is one of our highest recommended travel bags because it has tons of capacity and organization and a very comfortable back panel and straps.
The Outbreaker backpack from Tortuga is a solid 1-bag travel pick. I personally prefer the less bulky (and more spacious) experience of traveling with the Tortuga Setout, so be sure to check out that bag too. This Outbreaker, however, has quite a bit more padding and protection around the entire bag, so if you know you'll have delicate stuff inside you may prefer this one.
The Duffelpack from Peak Design expands from 45L to 65L (a little too large for carry-on on most airlines), has backpack straps that hide away, is made of weatherproof material and is a durable and organized companion for LOTS of different kinds of adventures.
The Black Hole Duffel from Patagonia is called the "black hole" because it fan fit all your stuff.