North American B-25 development (NA-40, P442, etc.)


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From the University of Washington's Aeronautical Laboratory in Seattle.

Obviously not an unbuilt project exactly but perhaps the final development stages between North American's NA-40B and the NA-62.

The first (1939) shows a NA-62 model with undercarriage down.

The model looks much like the first production B-25s except for the vertical tails. These surfaces have the outline and higher position of the NA-40 tails. Rudder shape is generally like that of the NA-40 but lacks the aerodynamic balance (à la the NA-62).

I include shots of the vertical tails of both the NA-62 (L) and NA-40B (R).

The second series show different approaches to the horizontal tail surfaces (the vertical tails appear to have the same proportions as those on the u/c down model).

On the left (1939), the horizontal surfaces are flat (and the wings are of constant dihedral). In the centre (1940), the horizontal surfaces have dihedral applied. On the right (1940), the horizontal surfaces are dihedraled and the vertical tails are also canted.

On the right, note also that dihedral has been eliminated from the outer wing panels (as per the tenth production B-25).

BTW: all these wind tunnel models models are identified as "Boeing B-25"

Finally comes the "North American Aviation XB-2B [sic] Pressurized Medium Bomber (1940)". Is it me or does this NA-63 have twin tails?


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"North American Aviation XB-2B [sic] Pressurized Medium Bomber (1940)" should probably read as the XB-28 which fit the decription and photo - IIRC both single and twin tails were considered before settling on a single fin.

Please see:
Thanks Hoo-2b-2day.

I didn't know that twin-fins were considered for the NA-63/XB-28.
I was cruising through some material in the Kuter papers at AFHRA today and came across North American's response to Air Corps Circular Proposal 39-640 (11 March 1939 w/3 amendments) dated 5 June 1939. The cognoscenti will recognize that proposal as the genesis of the B-25 Mitchell bomber, but NAA's response, Report NA-639, includes some surprises. North American examined multiple variations of what it called P442 (P for Project?) prior to the contract award. The familiar twin-tailed B-25 only first appeared as model P442-41. Wanna see what came earlier? Here's P442-17:


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Odd that NAA went from a single tail to twin-tail in their design, while Martin apparently did the opposite.
Smashing, Clioman! But where does the strange "Av'n Project" describer come from? A bit of franglais here?
Clioman said:
Ahh...Av'n is a contraction of Aviation. Sorry about that. ;)

Oh... silly me. :-[ I should have thought about that! (instead I got side-tracked by the French "avant-projet" which means "preliminary design"! :-\ ).
Anyway, I've modified your title to make it perfectly clear. ;)
I am unsure how the P442 is related to the NA-40 I found in two books: 'North American Aircraft 1934-1998 - Volume 1' by Norm Avery, Narkiewicz//Thompson 1998, 'B-25 Mitchell' by Jerry Scutts', Crowood 2001. As they present it, it is the ultimate B-25 predecessor that reached hardware status.

The NA-40 as first flown was powered by two Pratt & Whitney R-1830-S6C3-6 air-cooled radials.
When re-engined with two Wright R-2600-A71-3 air-cooled radials it became the NA-40B.

Images NA-40-01 and NA-40-02 are from the book by Jerry Scutts, all the other images and specifications are from Norm Avery's book.


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Topics merged.

Apart from the first image, the other pics you shared are of the modified NA-40B, with the glazed nose. This lone prototype was never actually purchased by the USAAC and retained its civilian registration X14221 until it crashed.

There were several stages in this prototype's development:
  • NA-40: full nose, Pratt & Whitney R-1830 engines
  • NA-40A: glazed nose, Wright R-2600 engines, glazed tail (project, not built)
  • NA-40B: glazed nose, Pratt & Whitney R-1830 engines (modified from NA-40)
  • NA-40B: glazed nose, 1,300 hp Wright R-2600-71 engines (same aircraft, also called NA-40-2)
Here are pictures of the prototype in its original NA-40 configuration and in successive NA-40B forms, all from Monografie Lotnicze No. 78, a painting of the NA-40B and a three-view arrangement of the unbuilt NA-40A (source unknown).


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Here are two more images from that proposal. The first one is's followed by something more familiar (P442-41)...


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Has nobpdy else noticed that the P442 is a considerably larger aircraft than NA-40/B-25 with wings that scream Northrop-Douglas in layout?


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Has someone seen this very strange single-tail modification of the Mitchell, and was this real of a photomontage? (it would be such a poor job of the latter that I'm inclined to think it could be real) I know the planned Super Strafer variant was to have a single-tail, but the markings seem to indicate this is earlier.

Sorry in advance if the question has already been asked and/or answered elsewhere.


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Just an educated guess, but I posit that this B-25 was a test mule for a single tail configuration, whether for the Super Strafer or as a mod for the B-25 production line. It is possible that as a test aircraft they wouldn't have bothered to over-paint the "meatball" in the center of the cocarde as the aircraft would never leave the U.S.

This B-25J "Mitchell" (44-30478) was converted to cosmetically appear as a A-20 Havoc.
fightingirish said:
This B-25J "Mitchell" (44-30478) was converted to cosmetically appear as a A-20 Havoc.

Thanks a lot, fightingirish!
From Flying magazine 1945-6.


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By 1944, NAA had made so many B-25s that the USAAF used them as advanced trainers for bomber crews. B-25s had largely replaced Beechcraft C-45 (civilian D18) as multi-engined trainers.
The RCAF continued using B-25s as pilot-trainers, bombardier-trainers, navigator-trainers and VIP transports until 1962.
By 1944, NAA had made so many B-25s that the USAAF used them as advanced trainers for bomber crews. B-25s had largely replaced Beechcraft C-45 (civilian D18) as multi-engined trainers.
The RCAF continued using B-25s as pilot-trainers, bombardier-trainers, navigator-trainers and VIP transports until 1962.
Post-war, the USAF used them as trainers, VIP transports, and officers hacks as well.

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