If you're planning to paint a large area in a single colour, for example on the B-17, whether in Olive Drab upper surfaces and Neutral Gray undresides, or bare metal overall, that's a heck of an area to cover evenly with a paint brush.
It
can be done without leaving brush marks, if using enamels not acrylics, but takes practice, or more correctly, experience and 'knowing' the paint and brushes being used.(I posted a basic guide to brush painting, some years ago, elsewhere in the modelling threads).
I started using an airbrush (with a propellant can !), just under 40 years ago specifically for two main types of paint finish - single colour over large areas, and mottled schemes on Luftwaffe aircraft - but still continued to use a paint brush for most other applications, and I still do on some models.
But again, I stress that I use enamels, which are more versatile than any acrylics, and allow more flexibility in use, providing a smoother, more even finish.
If you intend to paint a B-17, then I'd highly recommend using the airbrush, but first, practice, practice, practice, using the colour(s) to be used on the model, spraying onto a scrap model to a) get the feel of the brush over compound curves, joints and angles, and b) get accustomed to building-up the colour in thin coats, with a single pass of the airbrush in one direction.
It's better to apply two or more thin coats, than try to cover a large area in one go with a heavy coat, particularly with a single colour over a large area. The latter will only result in pooling of the paint, runs, drips, sags and all sorts of drying problems,
Using an airbrush is not the dark, mysterious science that some think it is - after all, young women in 'beauty salons' use them daily, to spray nail varnish !
It is a relatively simple, straightforward task, although, as in any form of 'artistic' work, it needs at least a little practice. The biggest problem in using an airbrush is the 'fear' of using it 'for real' for the first time - overcome that, and the problem no longer exists.
If the B-17 model is going to be sprayed, in whatever scheme then, as it's an important subject,with the 'family connection', then I'd suggest that you concentrate on getting the best finish possible, and leave weathering and other surface variations for another time. Better to have such a model looking good, although maybe 'factory fresh', than risk spoiling it by possibly overdoing the first weathering attempt.
However, just for interest, below is a pic of my (still unfinished) 1/48th scale B-17, built and painted around 24 years ago. This was brush-painted, as I didn't have an area in which to use an airbrush and compressor at the time, and still looks fairly reasonable as far as the finish goes, although 'some day' I might re-touch a couple of areas using the airbrush.