Article 9865 of rec.aviation.military:
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From: jfb@usgp1.ih.att.com (Joe Baugher)
Subject: Bomber Series--Morth American B-25 Mitchell (2 of 32)
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Summary: B-25
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Date: Thu, 5 Jan 1995 00:50:47 GMT
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Although the NA-40 was unsuccessful in gaining a production contract 
in the Army's light bomber competition, it nevertheless provided the 
inspiration for North American's entry in an entirely different 
competition.  

On March 11, 1939, the Air Corps issued Proposal No. 39-640 for the 
design of a medium bomber.  According to the specification, a bombload 
of 3000 pounds was to be carried over a range of 2000 miles at a top 
speed of over 300 mph.  The proposal called for either the Pratt & 
Whitney R-2800, the Wright R-2600, or the Wright R-3350.  Requests for 
proposals were widely circulated throughout the industry.  Proposals 
were received from Martin, Douglas, Stearman, and North American.  

Lee Atwood, North American's vice president and chief engineer, took 
charge of the medium bomber project.  North American's proposal was 
given the company designation of NA-62.  It drew heavily on the NA-40B 
design, and retained the basic format of that earlier design.  
However, the NA-62 was a somewhat larger airplane with greater speed, 
range, and payload capacity.  The wing area was some ten square feet 
larger than that of the NA-40, and the fuselage was six feet longer.  
Gross weight of the NA-62 was 28,000 pounds, as compared to 20,000 
pounds for the NA-40B.  In order to incorporate the increased bombload 
demanded by the new specification, the raised tandem cockpit of the 
NA-40B was replaced by a side-by-side cockpit with a top line that 
conformed with the top of the main fuselage.  This made for a wider 
fuselage, and the bombardier's passage to the nose was by a crawl 
tunnel underneath the flight deck.  The wing was lowered from the high 
shoulder position on the NA-40B to a mid-fuselage position, but it 
retained the continuous three-degree dihedral of the original design.  
An NACA 23017 airfoil was selected for the wing root, changing to a 
NACA 4409-R airfoil at the tip.  A slight camber reversal or "reflex" 
was incorporated at the tip trailing edge which moderated stall 
characteristics.  The nacelles of the Wright Cyclones were extended 
aft of the wing.  Each mainwheel retracted backwards into the engine 
nacelles.  

The crew was five--pilot, copilot, bombardier, navigator/radio 
operator, and gunner.  The navigator's compartment was directly behind 
the flight deck.  

The armament consisted of four 0.30-inch machine guns on flexible 
mounts.  There was a 0.30-inch machine gun in the nose which could be 
installed on any one of three ball-and-socket mounts cut into the nose 
(one pointing straight ahead and one to each side).  Another 0.30-inch 
flexible gun was mounted behind a Plexiglas hatch cut into the upper 
rear fuselage.  A third 0.30-inch machine gun had firing positions at 
each waist window and from a hole in the floor.  There was a 0.50-inch 
machine gun in the extreme tail, mounted in a streamlined transparent 
canopy.  The tail gunner lay prone in this position, the Plexiglas 
canopy featuring clamshell doors which opened sideways to permit 
traversal of the gun.  Maximum offensive load was 3600 pounds of 
bombs.  

The NA-62 was submitted in time to meet the Army's September 10 
deadline for the medium bomber competition.  The Army was sufficiently 
impressed with the North American proposal that on August 10, 1939, 
they issued a contract for 184 NA-62s under the designation B-25.  
This contract was finally approved on September 10.  At the same time, 
the competing Martin Model 179 was issued a contract for 201 examples 
under the designation B-26.  Since the design had been ordered "off 
the drawing board", there was no XB-25 as such.  

A full scale B-25 mockup was approved on November 9, 1939.  
Construction of the first B-25, serial number 40-2165, was in final 
assembly by the early summer of 1940.  The first flight took place on 
August 19, 1940, test pilot Vance Breese being at the controls and 
engineer Roy Ferren sitting in the right hand seat.  

Two of the early B-25s were delivered to Wright Field for tests, while 
the first B-25 was retained by North American.  Eight more B-25s were 
built with the original continuous wing dihedral, but Wright Field 
tests showed that this feature led to some directional instability, 
including the phenomenon of "Dutch roll".  In addition, the aircraft 
made banked rudder turns with such a configuration, which was 
unacceptable to the military, which required flat rudder turns when 
making making small heading corrections during the bomb run.  This was 
cured by adopting a cranked or "gull" wing, with the sections outboard 
of the engine nacelles being changed to horizontal.  This change cured 
the instability problem.  Exactly when this change was introduced is a 
matter of uncertainty.  It has been reported that this change was 
introduced on the tenth B-25 built, but there are no records to 
confirm this.  This "gull" wing configuration was retained throughout 
the entire Mitchell production run.  

The first few B-25s experimented with various geometries for the 
vertical fins.  The original configuration resembled that of the 
defunct NA-40B.  The fins were gradually enlarged and squared off, the 
second and third configurations being rather ungainly 
rectangular-shaped verticals.  After additional testing, a flattened 
triangular shaped was attempted before the final, back-tilted fin and 
rudder configuration was adopted.  

The idea of honoring General "Billy" Mitchell by naming the B-25 in
his memory was apparently the idea of Lee Atwood.  The Air Corps
readily agreed.

The first B-25 was accepted by the Army in February of 1941.  The 
first recipient of the B-25 was the 17th Bombardment Group based at 
McChord Field in the state of Washington, 19 examples ultimately being 
delivered.  One B-25 each was sent to Chanute and Lowry Fields, with
a couple being retained at the North American company.  A total of 
24 B-25s were built.  

Following the completion of its test series, the first B-25 (40-2165) 
was modified by North American Aviation as a company transport.  All 
military equipment was removed and seven seats were installed in the 
main fuselage.  The bomb bay was converted for baggage and a bunk was 
installed in the crawl space above the bomb bay.  Windows were cut 
into the sides of the fuselage and the greenhouse nose was completely 
faired over.  It crash-landed on January 8, 1945 during a routine 
check flight.  The crew was uninjured, but the aircraft was damaged 
beyond repair.  

In 1943, B-25 40-2168 was modified as General Arnold's personal 
airplane.  This plane was one of the nine constant-dihedral B-25s.  It 
had its wing panels revised to zero degree dihedral.  The 
modifications were otherwise identical to those carried out on 
40-2165.  The extent to which the General actually used this airplane 
are unknown.  After the war it was sold as surplus on the commercial 
market, passing through several owners (including Howard Hughes).  The 
plane remains in airworthy condition.  

Serials of B-25 Mitchell

40-2165/2188	North American B-25 Mitchell
			company numbers 62-2834/62-2857
			2165 modified as company transport
			2168 modified as General "Hap" Arnold's
			personal transport.  Later operated by
			Hughes as N75831.  Still airworthy a
			N2825B

Specification of North American B-25 Mitchell:  

Two Wright R-2600-9 Double Cyclone fourteen-cylinder air-cooled 
radials, each rated at 1700 hp for takeoff and 1500 hp at 2400 rpm.  
Performance:  Maximum speed 322 mph at 15,000 feet.  Service ceiling 
30,000 feet, range 2000 miles with 3000 pounds of bombs.  
Weights:  17,258 pounds empty, 28,557 pounds maximum.  
Dimensions:  Wingspan 67 feet 6.7 inches, length 54 feet 1 inches, 
height 16 feet 4 inches, wing area 610 square feet.  
Fuel:  two forward wing tanks, total 484 gallons.  Two rear wing tanks 
total 432 gallons.  One droppable bomb bay tank, 420 gallons.  
Armament:  one 0.30-inch machine gun in flexible mount in the nose.  
One 0.30-inch machine gun in a flexible dorsal position.  One 
0.30-inch machine gun in flexible waist position.  One 0.50-inch 
machine gun in flexible tail position.  

Serials:

   B-25 Mitchell: The Magnificent Medium, N. L. Avery, Phalanx, 1992.

   Medium with the Mostest--The B-25 Mitchell, Jerry Scutts,
   Air International, Vol. 44, Nos 2 and 3, 1993.

   Boston, Mitchell, and Liberator in Australian Service, Stewart
   Wilson, Aerospace Publications, 1992.

   Famous Bombers of the Second World War, William Green, Doubleday,
   1959.

   North American's Flying Gun--The Story of the B-25 From Paper
   Airplane to Legendary Bomber, Jack Dean, Wings, Vol 23 No 4, 1993.

   United States Military Aircraft Since 1909, Gordon Swanborough
   and Peter M. Bowers, Smithsonian, 1989.

   United States Navy Aircraft Since 1911, Gordon Swanborough and
   Peter M. Bowers, Naval Institute Press, 1990.

   North American B-25A-G Mitchell, Aircraft in Profile, Doubleday, 
   1966.

   Jane's American Fighting Aircraft of the 20th Century, Michael J.
   H. Taylor.


Joe Baugher			  **************************************
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