Will a 40L backpack fit under an airplane seat as a personal item?

It depends on how it's packed and the airline. Verified-purchase reviewers and r/onebag posts repeatedly note that softer 40L bags like the LOVEVOOK or Taygeer can be smashed under most domestic seats when partially full, but structured 40L packs like the Farpoint are usually overhead-bin territory. Budget carriers are the most strict.