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In general, duffel bags are not specifically designed for carry-on purposes. They are designed as a general purpose soft-sided case for just about anything.
You can find one that when filled (by this I mean, by volume) meets whatever size requirements there are for your arline, you can carry it on board - assuming you keep it below the allowed weight.
There are some duffel bags that are designed specifically as carryons; but these come in two main flavors.
The designer type, high quality material - do-only-one-thing duffel. This is basically a soft case with one (maybe two zip enclosures) and a handle to carry it; here is an example from a company that I have used before for bags called Waterfield Designs. The smaller one is what I would recommend for carry-on:
Then there are duffel-type bags that are specifically designed for carry-on travel; and these come with additional travel-friendly features (such as dedicated pockets for travel documents, and straps to carry the bag as a backpack).
Here is an example of one such bag, from Nomatic:
Any of the bags above will serve you well as carry-ons, but they are not secondary bags - they will not collapse and are designed to be your primary carry-on luggage item.
As the Nomatic is a kickstarter project and not yet available, there are some other alternatives.
The one I personally have is the Travel Pack from Aer. It is not strictly a duffel bag, but rather a flat-loading, expandable backpack - which was more practical for my tastes.
They also make the traditional duffel pack - a hybrid duffel and backpack and the more traditional gym duffel.
Upvote:4
Duffel bags generally have one thing in common - they are "soft" walled and flexible.
The requirements airline publish as the size that the bag needs to be at the time you take it onto the plane - not the maximum size that the bag could be at it's extremes. Presuming your bag, at the time you're boarding, could be made fit within the size requirements (or within the sizer that is frequently available at the gate) then you should be allowed to take it onto the plane.
Thus most duffel bags less than 22" will be suitable for carry-on bags, as long as you only fill them enough that they can be moulded to be below the 22" x 15" x 8" size requirements (for whatever the airline you're flying allows).
If you were to fill the bag so that it can not be compacted to below the 8" 'depth', then it would likely not be allowed on-board.