Holding model aircraft for airbrushing

Wear latex or nitrile gloves. This protects you and the model. Just be sure to rinse away any powder that they are treated with to prevent sticking while in the package.

Manstein's revenge

...my hands...

 That's my first thought!  For one colored aircraft, I'll use the wing, sometimes tail, to hold on to. After about 20 minutes or so, the paint is dry enough to hold again, go back and paint the rest.

Here are some ideas

Hold That Model

Hold That Model Pt I

Hold That Model Pt II

Hold That Model Pt III

For a model as big as the B-29, consider painting it in sections. Leave the tail and outer wing sections for last. Mask to protect them from overspray and use that as your "handle".  Otherwise ParaFilm offers a great way to protect already painted areas while you handle the model.

With single engine it is easy- I put prop on last, and use prop mounting hole for a dowel or metal rod.  For multi-engine it is indeed a problem.  One solution I have found on some models is to paint in two sessions- say all but left wing first, then left wing after first areas dry.  A bit of a challenge to match at join line, but certainly not impossible.

...my hands...

You could use a strong wire coat hanger or a bit of fencing wire. A bit difficult to explain, but straighten it out, put a 90 degree bend in it, lay it flat on the table, about 6>8 inches away from the tip of the bend, bend the wire straight up & then bend the last few inches of this to the horizontal. The horizontals can the be stuck into intakes, jet pipes, nacelles ...... to support the model.

It's pretty handy, as it can be bent into whatever shape required to support the model - If you want to to topside & underside in one session, just fit two supports & you can turn the subject over without touching it.

Can the bomb bay doors be left off so a dowel can be secured inside temporarily to hold for the painting process?

If the bottom of the aircraft is a different color to be masked it can be simply set on a angled turn table to rotate without handling, most choose to leave fiddly bits like landing gear, wing spars, pilot tubes off until the main components are painted to avoid damage and touch them up later or pre paint them.

With several options its really a decision you have as to the approach you take, its best to have this planned out in the initial reading of the instructions. Deviation of said instructions to accommodate ease of painting is preferred over problems further in the build.

Hack through the painting to completion, regardless it will be a lesson learned for your next build good or bad.