There are probably hundreds of different ways to weather an aircraft. Some are best done during the painting process (pre-shading, color depth, blending filters), or after decals go on, or at the end of a build. A few I'm a fan of:
I'm not really using it any more as I've become a big fan of shading in multiple colors and building up the final coat in thin layers, but for several builds I used a three-layer blend technique that imparts a sense of wear and adds depth to the paint - you can find the technique here under Painting.
For the already painted and decaled model, you can use salt fading. Make sure you've got a good, water-proof clear coat in place, soak the thing with warm water from a spray bottle, then grind salt onto it:
Mix up a very thin (more or less transparent) mix of a light shade paint - a light, slightly brownish gray works well - and spray it very lightly over the salt. Then rinse the salt off, reapply, and go with a darker "grime" mix. Rinse that off. Oh, and don't use Tamiya paints with the salt since they stain out for some reason. Final result is a nice, randomly worn paint finish:
You can also use washes, as suggested, HOWEVER if you're talking B-17 I'm going to assume it's a 1/48 Revellogram, which means raised panel lines, that in my experience don't play nice with washes. You can always do some post-shading work, but that's a scary thing if you're not confident with the airbrush.
You could do some post-filtering. Mix up (again, very) thin renditions of the main color, but with a drop of gray or dark green or what have you, and go back very gently over certain panels and areas with them. Make sure the control surfaces are lighter than the metal wings as the doped fabric tended to fade out faster. A very light mist of your base color over decals can reduce their contrast and make the entire thing seem more worn, as well.
There's also chipping along the wing leading edges, major panel lines that could catch in the slipstream or that saw a lot of maintenance work. As well as the prop blades. Lots of ways to do this, mainly drybrushing silver paint. But my preferred method is "dotting" with a prismacolor silver pencil. Just tap-tap-tap it against the surface to build up miniscule "chips" that can be built up and up to represent worn areas.
And exhaust stains of course...pastels or pigments work well here. You'll probably want more of a grimy gray/brown than straight up black, which is more common on Luftwaffe or Soviet VVS machines.
