There are many, many camera bags to choose from, but if you need one that is optimized for travel photography (especially airline travel and its often ludicrous requirements), read on for a very inexpensive option.
It’s got a hell of a long name, but the “Abonnyc Photo Hatchback 19L Camera Backpack – Waterproof Anti-shock Daypack Style Backpack For DSLR SLR and Mirrorless Cameras , Dark Blue” is one heck of a value at $70.
Travel Photography Camera Bag Overview
The overall ergonomic design is excellent. It looks like a fairly standard backpack, but the way it’s laid out is totally optimized for photography gear. A large chunk of the interior space is allocated to a customizable “cube” that holds lenses and bodies. This cube is also slides out completely for easy access and is advertised to be “waterproof”. I’ve not tested this, but it seems to be at least a plausible claim. 
This gear cube unzips from the rear of the backpack, but on the side that actually faces the wearer’s back. This puts more space between the outer parts of the bag and your gear, keeping it safer from bumps, drops, etc.
The non-gear storage areas are fairly small and sparse, but there is room for a 15″ laptop and a few small items in the front of the bag. There is also a netting on the sides to hold bottled water or other similar sized items.
If that weren’t enough, it also comes with a full-cover rain fly to protect the entire bag in a rain storm.
Airline Sizing
While I do like the way it’s laid out, my favorite aspect of the Abonnyc Photo Hatchback 19L is that when laid flat, it will fit under most airline seats. This makes it qualify as a “personal item” instead of a full-fledged “carryon”. I’ve taken this bag on several flights in the year that I’ve owned it, and so far it’s fit under all seats (American, Delta, Jet Blue, Spirit).
This means you can keep your gear as close to you as possible in transit and not have to worry about every other idiot on your flight cramming their 19th-century steamer trunk into the overhead and smashing your bag.
It might also help you avoid a checked bag and the associated egregious fee.
Gear Capacity
Depending on the size of your lenses and bodies, you can reasonably expect to fit two bodies and 3 or 4 lenses, along with a flash and TTL controllers, a laptop, and small items like filters, caps, cleaning clots, memory cards, and even a small first-aid kit (I never travel without one). With careful packing, you can get a lot of gear in this thing.
The Verdict
For about $70, it’s really hard to beat the Abonnyc Photo Hatchback 19L for an airline-friendly travel photography camera bag. It’s compact but holds a lot and fits under most airline seats.