First off, what would you be bugging out from? What event would cause you to have to leave home? Do you have a plan as to where you would go and how you will get there?
Planning a BOB is a lot more involved than just throwing three days of supplies in a bag and heading out to the woods. If your BOB is the sole item you will carry with you in the event of needing to bug out (AKA Last resort...) then you should plan for the 5 C's of survival. Cutting, Cordage, Cover, Combustion and Container. Food and water should be planned out to be as lightweight as possible and plan to scavenge along the way.
If your BOB is to compliment other preps in a vehicle, then you should plan your system accordingly.
I'll agree with the statement of assemble the gear you want to take with you and then get a pack. I will disagree with the "tactical" perspective. A backpack does not make you "tactical". Even a simple hiking pack will make you look tactical if you have tactical looking things (I.E: Knives, water canteens etc) strapped to the outside.
A MOLLE backpack doesn't scream tactical. IMO a camo hiking pack can scream tactical for anyone looking for people with camo stuff....
My BOB is a RUSH72. It's in the closet ready to go in the event of an emergency. Speaking of... I had best look at that bag tonight, it's been a while since I've gone through it.
My 24 hour kit/GHB is a RUSH24 and it sits in my truck. It's black, doesn't have stuff strapped to it, but carries enough gear to get me home and stay overnight somewhere if I need to.
If you are bugging out, or even trying to get home and you strap a big hiking bag to your back, people are going to look at you funny and may even pick up on the fact that you are well prepared and WILL attempt to take your goods. A small 24hr bag well stocked might lend to the notion you have books and maybe a half full water bottle in your bag, not three days worth of supplies.
Your BOB should consist of items that will not only make surviving an emergency easier, but items that will allow you to outlast your initial food and water contents. You may need to source water from a stream or lake. You may need to catch game to eat. Lots of things to consider.
As for the bag itself, consider the old adage "Buy once, cry once". Buy a high quality bag that will suit your needs, not something cheap that you will need to replace in a year's time.
A pack I discovered today looks ideal. Its the Paratus 3 day operator's pack. Im kind of leaning towards that one now. Its sub $100.
Size and shape is up to you. Brands that I like enough to buy more than one of are Camelbak, Eberlestock, Hazard 4 and Maxpedition. The first two offer more traditional styling and have deep roots in the serious outdoor sports markets. The second two tend to have a butt load of molle, velcro fields and look painfully tacticool but are up to some serious hard use. Of all my bags the Eberlestock Destroyer is my top camping/SHTF pack and the Hazard 4 Takedown is my shooters MVP.
OP, as you can see there is a huge degree of difference when this question gets asked. I don't blame you for seeking advice. I'll add in my own two cents as well with my own question. What do you carry on your person every day as a matter of course? Wallet, keys, phone, pocketknife, etc. This all forms your Everyday Carry (EDC) supplies. There is a lot one can carry on them to help out with a SHTF situation. For instance I rarely go anywhere without my pocketknife, wallet, phone, and several other things. They're things I use a lot and many have multiple uses as well.
Obviously it's rather impractical to keep a three day supply of food on your person at all times, hence the BOB. However you can reduce what you carry in a BOB with your EDC. If it's already on you and you're used to carrying it why put it in a BOB. When it comes to what specific kind of backpack I would agree with those who say that regardless of what you get, make sure it's of high quality even if that means spending some more money. If you have a backpack to use to at least test out what you think you might want before you get your high quality pack that might help too. That way you can see what you truly need, what you don't need, what you'll use, etc.
I agree there is a lot more military looking bags, clothing being worn used now days . Still not a big fan having used both.
Civilian backpacking gear is much lighter/ comfortable. Look at joining a local backpacking group many members will have extra gear to let you barrow. This will do several things. 1. get out and learn/ practice camping, traveling by foot . Don't know how to use a stove trust me they will be glad to teach you and show you several stoves. Same with map compass, fire starting . 2. lets you try or at least see different brands of gear. 3. put you with folks that might have used gear to buy so you can save money. 4. FUN.
As far as what to take - what do you need to take with you on a week backpack trip . That is what you need plus in an uncertain security situation some type of self defense /at least a small hand gun or pepper spray/ and a gun shot kit /Quick clot, tourniquet, combat bandage/ .
You can only carry so much stuff regardless of what pack you have or how light weight your equipment- physical conditioning will up that number but you will still want more than you can carry. The AR 15 with 800 rounds sounds good till you carry it for 5 miles.