Article 9821 of rec.aviation.military:
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From: jfb@usgp1.ih.att.com (Joe Baugher)
Subject: Bomber Series--North American B-25 Mitchell (1 of 32)
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Summary: NA-40 
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Date: Wed, 4 Jan 1995 02:09:33 GMT
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The North American B-25 Mitchell is rated by most as being the best 
all-round light-medium bomber of World War 2.  It was operationally 
efficient, docile, adaptable, and had an excellent all-round 
performance and particularly good handling characteristics.  A total 
of 9816 Mitchells were built, greater than any other American 
twin-engined bomber.  It served with the United States Army Air Forces 
(USAAF) on virtually all fronts during the War, and it also served in 
significant numbers with the United States Navy.  It was also an 
important component of Lend-Lease, being supplied in large numbers to 
the Royal Air Force, the Soviet Union, Holland, Australia, and Brazil.  

The B-25 is one of the few American military aircraft to have been 
named for a person.  The aircraft carries the name of Colonel William 
C. "Billy" Mitchell (1879-1936), an Army officer of the early 1920s 
who had been the assistant Chief of the Army Air Service.  He was 
court-martialed for insubordination in 1925 as a result of his 
outspoken views on the future of air power.  His views were ultimately 
vindicated, and he was posthumously promoted to Bragidier General.  

Even if the Mitchell had never done anything else in its career, it 
would have achieved immortality for its most famous mission, the 
Doolittle Raid on Japan in 1942.  

After the war, the Mitchell soldiered on in training roles with the 
USAF for many years, the last example not being retired until 
1959.  They served with other air forces even longer, remaining flying 
into the late 1970s with some Latin American air forces.  A number of 
Mitchells are still flying today on the international air show 
circuit.  

The origin of the B-25 Mitchell can be traced back to 1937, when the 
Army's Materiel Division began to investigate the possibility of the 
development of a twin-engined attack bomber with a performance that 
would greatly exceed that of the single-engined types that were 
currently in service.  In March 1938, the Air Corps issued Circular 
Proposal Number 38-385 that defined the requirements.  Payload was to 
be 1200 pounds, and range was to be 1200 miles at speeds greater than 
200 mph.  

Proposals were submitted by Bell, Boeing-Stearman, Douglas, Martin, 
and North American.  Bell's Model 9 proposal called for an aircraft 
powered by two liquid-cooled Allison engines.  It was withdrawn from 
the competition before anything could be built.  Boeing-Stearman 
submitted its Model X-100, a three-seat high-winged monoplane powered 
by a pair of untried Pratt & Whitney R-2180 radials.  Martin submitted 
its Model 139, a twin-engined mid-wing monoplane.  The Douglas entry 
was the Model 7B, a high-winged monoplane powered by a pair of 1100 hp 
Pratt & Whitney R-1830 Twin Wasp radials.  

The North American entry was designated NA-40 by the company.  It had 
a crew of five--pilot, copilot, bombardier/navigator, radio 
operator/gunner, and gunner.  At that time, a crew of five was 
considered rather large for an aircraft that was intended for the 
attack role.  The pilot and copilot were seated in tandem under a long 
slender canopy, an arrangement that minimized frontal area and 
permitted the use of a narrow fuselage.  A greenhouse-type nose housed 
the bombardier/navigator, and the radio operator and gunner were 
seated in the aft fuselage.  The cantilever wings were 
shoulder-mounted on the upper fuselage, and had a fixed dihedral of 3 
degrees 23 minutes.  The engines were mounted in underhung nacelles.  
The engines were a pair of Pratt & Whitney R-1830-S6C3-6 air-cooled 
radials rated at 1100 hp each.  They drove a pair of twelve-foot, 
three-bladed Curtiss electric propellers.  The armament consted of 
three flexible, hand-held 0.30-inch machine guns, one ball-mounted in 
the nose, one in a dorsal turret behind the cockpit canopy, and one 
that was moveable between waist and ventral positions cut into the 
sides and bottom of the fuselage.  Bays for two fixed 0.30-inch guns 
were provided in each wing, but these were not actually installed.  
Twin fins and rudders were fitted, and the aircraft was provided with 
a retractable tricycle nosewheel undercarriage.  The mainwheels 
retracted into the rear of the underslung engine nacelles.  

The NA-40 flew for the first time on January 29, 1939, company test 
pilot Paul Balfour being at the controls.  It carried the civilian 
registration NX14221.  On the early test flights, the aircraft 
experienced severe tail shaking which got worse as the speed 
increased.  In addition, erratic oil and cylinder head temperatures 
were recorded.  Subsequently, elbows and long tail pipes were fitted 
to the exhaust stacks and a squared fairing was mounted on the nacelle 
trailing edge Some improvements resulted, but the basic problems 
persisted.  

Maximum speed was a respectable if not spectacular 268 mph at 5000 
feet.  There was a general impression that the NA-40 was seriously 
underpowered, and in late February 1939, the Pratt & Whitney engines 
were replaced by Wright R-2600-A71-3 Double Cyclone fourteen-cylinder 
air-cooled radial engines rated at 1600 hp each.  The more powerful 
Wright engines were provided with more streamlined cowlings.  The 
over-wing exhausts were replaced by collector rings that were ported 
to exhausts outboard of the nacelles and the carburetor intake scoops 
were moved slightly aft.  The aircraft was redesignated NA-40B.  
NA-40B took off on its maiden flight on March 1, 1939, Paul Balfour 
again being at the controls.  The airplane had a considerable 
improvement in performance, with a maximum speed of 287 mph at 5000 
feet.  The aircraft was delivered to Wright Field in Ohio on March 12.  
The flight tests at Wright Field turned up very few problems, with 
virtually the only complaint being the slow feathering characteristics 
of the Curtiss electric propellers delaying the engine-out testing.  A 
shutdown engine could be simulated by idling the engine with the 
propeller in full high pitch.  

On April 11, the NA-40B crashed near Wright Field when the pilot lost 
control of the aircraft during an engine-out test.  Although the crew 
escaped serious injury, the NA-40B was destroyed by fire in the crash.  

Although the basic design of the NA-40B was deemed not to be at fault 
in the crash, the Air Corps nevertheless decided not to order the 
NA-40B into production.  The Army's attack bomber contract went 
instead to the Douglas entry, the Army ordering in July of 1939 123 
examples of a revised version of the Model 7B, the DB-7, under the 
designation A-20.  

Since the NA-40 was never ordered by the Army, it never acquired a
USAAC serial number.  

Specification of North American NA-40B:  

Two Wright R-2600-A71-3 Double Cyclone engines rated at 1500 hp each 
for takeoff and 1275 hp at 12,000 feet.  Dimensions:  Length 47 feet 
10 inches, wingspan 66 feet 0 inches, wing area 599 square feet.  
Weights:  13,961 pounds empty, 19,741 pounds loaded.  Maximum speed:  
309 mph at 14,000 feet.  Design range of 1200 miles with a 2100 pound 
bomb load.  Armament:  Three 0.30-inch flexible machine guns in nose, 
top turret, and an aft position portable between waist and ventral 
locations.  Two fixed 0.30-inch guns could be carried in each wing.  

Sources:

   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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