Originally Posted by RichP

I think you're going to struggle with fitting a 40l backpack under the seat when it's loaded with a laptop and a surface, and anything else.
I use a 5.11 Rush 48, which is in the region of 40l, has two side pockets for waterbottles, decent organisation, some molle on the outside, and is comfortable to lug around when it's full. I can pack a laptop and iPad in the compartment. But it'll never fit under the seat...

Gorucks are awesome bags, and may well be what you're looking for.