Backpack for two week trip that can be put in overhead compartment

Osprey far point 40 is very good. Osprey offer many different bags, but I have never had difficulty carrying this one on any planes here and it is made very well.

Backpack style travel bags are sold by Rick that work well. Go to REI or other reputable outdoor gear store and look about. Locks? Most backpacks have zippers that can be locked together with a small lock or I often just use an odd color of zip ties that may ward off some thieves. Have never had a problem, but then I use common sense in normal travels and amp it up when around crowds.. Backpacks are made of nylon generally or other soft material that's easy to slice open. As for cutting straps and stealing the bags, I guess that's a threat, but WAY down the list of potential problems from my perspective. Use a money belt to store important items.

This backpack here in the RS Store works great for me. Unless it's crammed full it form-fits to overhead bins. Locks easily fit in the zipper holes to secure various openings and pockets. Scissors would not be able to cut the straps or any part of the bag without real effort.

Keep in mind there is no such thing as a thief-proof bag. The best defense against them is awareness, common sense, and a money belt. Some people don't put locks on their bags because they believe it signals to a thief that there is something valuable inside.

Thank you for the suggestions everyone! I am shopping around and looking at all of these options now.

Also, it would be great to have a backpack that would be protected
from thieves.

As Bruce mentioned, it's just good practice never to stow anything of real value in a backpack. Besides backpacks not being allowed into all attractions, it's a good idea to keep your cards, cash and passports separate from your clothes and other non-essentials. There are any number of ways to do that: money belts, neck wallets, Pacsafe/Travelon shoulder bags, specially designed interior pockets, etc. What's important is that you do not have everything in one bag in case it's lost or stolen.

Clothes and toiletries can be easily replaced: passports and credit cards are a little more problematic so stow those ON you and don't worry about locking your backpack.

As long as the backpack is 22" or shorter, you can carry it on a plane. I recommend trying to stay around 20" give or take. 40 liters is a good size. A smaller person or lighter packer could go 35 liters. Everyone packs differently. I recommend packing cubes in bright colors (less likely to leave behind). I use ebags.com - classic ebags cubes and slimline. But, there's lots of brands. Osprey does make good bags. My favorite bag is Ebags brand - 2.0 etech JUNIOR bag. I am shorter than 5'5" and this bag is great for me. My tall husband could easily use the regular etech bag. Ebags also has a mother lode bag that is popular and well made. Good prices. I simply use a small combination luggage lock on the zipper pulls to seal the bag. Most bags don't provide the actual lock. Rather, zipper pulls that can be locked together or zipper pulls that intertwine and can be locked. Have fun!

The best bag I have found, by far, is the Tristar convertible bag by Tom Bihn at tombihn.com . It measures 19"x13"x8".

It is rugged, well designed, and good looking, and it carries well with either handle, backpack straps, or the ultimate shoulder strap. It is not cheap, but the external pockets keep everything organized and accessible. You can order (at additional cost) the Cache which snaps into the center compartment to protect your laptop. I find that the Eagle Creek ultra light packing cubes are a better value for keeping things organized within the pack. The Tristar came with metal zipper pulls, but I cut them off and installed the nylon corded pulls which were included with the pack. It holds 2000 cubic inches of stuff. If you need more, you can step up to the Aeronaut, at 2700 cubic inches, which is actually cheaper, but it is bigger.

I have traveled several times to Europe with this bag, and as a personal item I selected the Tom Bihn "Co-Pilot" shoulder bag. Same quality, same high price point, same durability, but serious luggage for serious travelers. If this sounds like a commercial, I apologize, but I love the bags. Lifetime warranties on both.

One important note. If you are using the bag as a backpack, take off the shoulder strap. I left the shoulder strap connected while using it as a backpack as I walked across the Promenade de Anglais in Nice after getting out of a taxi. I accidentally stepped through the dangling shoulder strap and tripped in the middle of the street, landing hard enough to break my wrist. I soldiered on, however, binding my swollen forearm with a velcro fastening nylon brace and fortifying myself with copious quantities of wine and cognac. Broken bones will heal (sometimes with a little surgery) but memories are forever, especially memories of France and other exciting places.

I like Red Oxx gear. They have 3 backpack styles which may suit your needs. The Skytrain opens like a suitcase and is 20x9x13. The C-Ruck (18x12x7) and Roadster Mini-Ruck (17x10x6) are like an actual ruck sack and have outside pockets as well. Red Oxx has a lifetime warranty so you will not wear these out. You can find them at Redoxx.com. Good luck!

Not all airlines allow 22 inches bags. Always check with every airline you will use this trip. I believe PacSafe makes a backpack that I am sure they will convince you is cut and theft proof. Their products have little clip latches on every zipper. I am referencing them...not recommending them. Some people love them I say they are overkill. Plenty of threads about the various products. You want a second item to comply with day use and museum allowance. For example, the Prado museum requires all back packs and bags bigger than 40 x 40 cm to be checked. Use that as a conservative guide. A crossbody bag with top zip and a latchable flap is the best foil for pickpockets IMHO.