If the flight attendant yells at me as it actually fits. But I think it does. Final verdict yes you can fit it it is a little snug on a foot roof.
Will a 35L backpack fit under an airplane seat?
A: Yes it will fit underneath an airline seat. A: it depends on the airline. For southwest, it's allowable as a carryon but not as a personal item that fits under a seat. They state that it must fit 16.25” (L) + 13.5” (W) + 8” (H).