Dumb, simple (I think) carpentry question -
I’m putting together a box made of 2x12 boards. The boards, eyed the long way, aren’t perfectly flat, of course, and when I assemble the box, the boards bow out and the edges aren’t flush with the cross-pieces. I was kinda hoping sheer force of the screws would ‘persuade’ the boards flat, but I guess my screwdriving prowess is no match for “two-by” board.
So is there an easy way to flatten the boards to get a nice, snugly fitting box?
Thanks.
Get some different boards.
If screwing doesn’t do the trick, the boards’ll fight you even if you steam them to softness and let them dry after assembly.
Use Long Screws at least 3" and a Power Driver. Ensure you pilot drill the boards.
Start screwing at the Bow and use many screws. Let it sit for a day and then tighten them manually and complete the screwing process. This will usually work on Pine. It probably will not work on Hardwood.
Good Luck,
Jim
Or get a planer and plane them flat.
Let the painter fix it. 
Just a joke about the different trades…
Both Squink and What Exit? are correct. While you might be able to get it all a little bit more square, the wood will always fight you. Best to start out square than to to try to make it square after the fact. When buying lumber, I always check the wood, picking thru it if I have to. Some things, like building forms, don’t always need the best lumber. Framing with bowed pieces can be done, as long as you know how to make the bows work for you. Building a box, like a planter box or a hope chest or perhaps making a porch or deck? Get the straightest wood you can find. I can eyeball it in three different planes and that’s generally a good enough method.
Buy a better grade of lumber…
Still gotta check each board. Just pick up an end and eyeball down it, looking for bows and swells. Turn 90 degrees, and look again. If it’s nasty, don’t buy it.
There is always a wide variance of quality within any graded bundle. You’re after the best. I usually start with a lower grade and see if I can get the pieces I need for cheap. I’ll buy long cheap boards that are warped or damaged on the end so I can use the middle.
Picking your material is the second step in any woodworking project, and requires just as much skill as any other process.
You can dress the edges of the boxes so they match using a router table. The sides will still bow, but the joints will be clean, if odd looking.
These suggestions are all well and good, but they would have poor bup start from scratch, at some expense. When you say 2 x 12, I’m imagining that you’re using ordinary framing lumber, correct? So it’s a soft wood that can easily be bowed into place and held there by screws. The trick is to push the wood home, so it actually meets the crosspiece, before driving the screw, using a power drill with a screw bit. You might want to turn the assembly on its side and stand on it, if possible, so your weight forces the wood against the crosspiece. Then drive in a 1 x 3 drywall screw. If you want, you can lubricate it first with a bit of oil. No need to drill pilot holes if we’re really talking about a spruce 2 x 12.
Rather than asking mechanical fasteners to try to pull the cup out of the boards, I’d coat the mating faces with an aggressive adhesive for wood such as Gorilla Glue, and then use bar or pipe clamps with scraps to preclude clamp marks on the finished project. You can apply much more force with clamps than simple wood screws could ever exert. After the glue has dried, you could drill, screw, and plug the holes if desired.
Bar Clamps are the best for the job. When Gluing up 9’ oars we use 10+ clamps to epoxy them together. Not sure why, but I was thinking Bup probably doesn’t have a half dozen Bar clamps.
Jim
In the trade, we call this condition “cupping”. (I am a carpenter/remodeler)
With a softwood like pine or doug fir, the above-mentioned tips might work for a while, but eventually the material will “creep” back to the cupped shape or at least crack. Or you will strip the screws.
And believe me you will need badass clamps/leverage to take a half inch or more (common in that width) cup out of a 2x12.
Hardwoods, forget it.
And NCB, I gear you on the painters, but I AM the dang painter. 
I don’ t know, I’m imagining that you’d be trying to glue to the end grain of the cross member, which is not the way to go here. And like What Exit?, I’m not picturing bup as having a couple of bar clamps handy – hence my advice to take the curve out of the wood by standing on it while driving the screws.
And Klaatu, this doesn’t sound like cupping. From the OP, it sounds like the wood is out of true along its length. Of course, if there were cupping too, I wouldn’t be surprised.
Maybe bup could tell us the kind of wood, and also about the nature of the joint between the out-of-true piece and the cross member.
So am I! That’s my primary trade, anyways. In addition to everything else I do (like Home Depot At Home Services), I just got hired on (contract basis) as a punch guy for a commercial builder. Yeah, I’m no Dances, but I’ve had to learn quite a few trades in the past 20 or some odd years.
So, I was just throwing out a little joke people in the residential industry might catch. 