We did some 3' raised beds with 1" wood and only the 5' sections bowed. The 4' beds were fine a couple years later.

My crappy raised bed has 2" lumber because I used old waterbed sides for it.

Painting the sides? Even in dry Sacramento the soil will be constantly wet in there. Maybe waterproofing?

Okay, so we went with 2" thick fir and the bed will be 12' by 4'. Because we decided on 2" thick boards, I'm only putting one support post (4x4 fir) down the length of the 12' side.

The corner posts are going to go underground about 9" and the side support posts will go in only 6". A "helpful" observer suggested putting gravel around the posts rather than concrete. I had originally planned just to use dirt. What do you think? I don't think gravel will provide any support at all, but it might help with drainage around the posts which would delay them rotting.

Use treated posts? I'd use soil in the post holes.

If you're worried about your corner support structure dig down deeper than 9" (why so little?). The deeper it is the more 4 by 4 surface area there is to apply counter pressure.

If frost is a consideration add 6" or so of clear gravel to the bottom of your post hole. This will act as a drainage area that will allow for expansion during periods of frost.

Soil back fill will be fine, just pack well at every 4" or so. Concrete certainly cant hurt but may be considered overkill.

Use pressure treated fir or pine and forget about rot. If non PT you should still get at least 25 years out of your posts. You can always spray a wood preservative on the buried portion for added protection.

You will need the 4 corner 4x4 posts and a mid way point 4x4 at the 12 foot span.
Use 2x12's redwood for your sidewalls.
1x12 will bow and split in a matter of hours.
Redwood is preferable over fir due to it's rot resistant qualities.

The raised beds in the photo below are 10 years old. They are holding up fine with the exception of one 4x4 post rotting away due to a leaky irrigation pipe.

The 4x4 redwood posts extended into the ground by 6 inches and were set in a modest sized concrete footing.

No angle bracket were used. The longest raised box is 6 feet long.
The foreground boxes are 3x3.

I would suggest using carriage bolts or a high grade deck screw with a decent size washer to secure your 2x12 to the 4x4.

You'll get about 15 year , possibly 20 years out of the boxes if constructed out of redwood. {{gwi:50068}}From Raised Garden Beds

Have you ever thought about using galvinized roofing. I have used this. And with age it realy looks good, even better if you can find used miteral. Use 4x4 post 4 ft o.c. run the roofing virdical cut to any hight you want. it is very strong.