The simplest & best way to find this out to me is in the photo of Dutch.
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Now based off the info the nephew collected about his uncle he has done his home work but has hit a brick wall.
http://home.mchsi.com/~anderson.kevin/roy_powerhouse.html http://forum.armyairforces.com/Update-on-the-B25-Powerhouse-m104947.aspxI think based off what I can ascertain from his web page the pictures of Powerhouse came from his cousins. So the original photo of Dutch above is from a fellow crew member Donald C Robertson who left them to his son. I have tried to blow this picture up larger & it is just to messy to get a read of the tail number in the back ground.
So you need to try & get the original photo or even better the negative & see if you can get a clearer photo of the plane in the back ground & I think Mr Robertson JR has the answer to finding the tail # to Powerhouse.
How do I come to this conclusion? Well here we go.
(PLEASE NOTE I AM NOT A PHOTO SHOP KIND OF GUY SO YOU MAY HAVE TO SAVE THESE PHOTOS & PLAY WITH THEM TO SEE WHAT I AM SEEING)
If you note in this first picture in the back ground you can see a small tent & you can see the canvas & tent polls as well as the rope coming off the poles most likely pegged in to the ground.
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Now in this photo note the forked tree & what looks like materials of some kind tool boxes maybe? & possibly a kneeling mechanic as well as one sitting besides him under the aircraft near the rear ladder of the plane. Now take a look at the right lower engine cowling it is not there & also there seems to be an access panel removed to the engine nacelle. Clearly this aircraft is being worked on.
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OK now lets look at one of the close up pictures of Dutch.
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Now if you look to the left you can see the same forked tree with the tail of a B25 in the back ground. Shame the fellow in the white hat with the cigar is not a foot or so to the right as he is hiding your tail number. Now look at this picture of the lower right side of the above photo looking over the top of the right wheel of the plane.
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In my opinion that is the canvas of the tent as well as its tent pole & supporting equipment from the first photo.
So if I am correct these two photos were taken at about the same time but from two differant positions by the same photographer on the same day. The fact that the same tree and two B25's & a tent as well as two of the same people in the same clothes all in two differant photos is just to coincidental.
Now there is a third picture of Dutch. Not a lot to go on other than similar trees in the back ground same man in all four pictures & the sun is out. Where in the other three it must have been overcast at the time as there is not much in the way of shadows.
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OK so now lets get to Powerhouse & how I think this all ties in.
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As you can see the trees in the back ground look some what the same also there appears to be tool boxes & equipment under the plane again clearly some one is working on this plane as in the first photo of Dutch. Now in the first photo you can see the canvas tarps are on the top turret as well as the cockpit area of the plane. Not much point in them being there if you want a picture of some one sitting in the cockpit with it's cool nose art right?
Now to me this is the clincher

on the ground notice the square shaped material with a round like curve to it.
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I think that is the access panel to the right engine nacelle of the plane in the back ground of Dutch in the first photo.
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So in conclusion I believe all these photos were taken at the same time by the same photographer & if that little bit of skin is the access panel it is just to much evidence to be coincidental & therefore I think you have a picture complete of Powerhouse. All you need is a better photo to get that tail #.
Let the debate begin.

I did the same thing trying to zoom sharpen sharpen sharpen the tail on the background plane last night. I think I can make out the last 4 numbers 2549. I think you are correct in you deductions and the times the pictures were taken. 2549 does not match up on the list of planes the fellow lists though.
Ok...I was just looking at this photo in PSP...it almost looks like a tail number of 5544, or 2544 (those are only the last four digits of it, and a bit of a guess).
and a good guess I think, very similar to what I got.