The North American B-25 Mitchell
On 11th March, 1939, the Air Corps issued Proposal No. 39-640 for the design of a medium bomber. According to the specification, a bomb-load of 3000 pounds (1361 kilos) was to be carried over a range of 2000 miles (3218 km) at a top speed of over 300 mph (483 km/h). 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 aeroplane with greater speed, range, and payload capacity. The wing area was some ten square feet (0.90 sq. meters) larger than that of the NA-40, and the fuselage was six feet longer (1.83 meters). Gross weight of the NA-62 was 28,000 pounds (12701 kilos), as compared to 20,000 pounds (9072 kilos) for the NA-40B. In order to incorporate the increased bomb-load 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 (The upward or downward inclination of an aircraft wing from true horizontal) 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 main wheel retracted backwards into the engine nacelles.
The crew was five -- pilot, co-pilot, bombardier, navigator/radio operator, and gunner. The navigator's compartment was directly behind the flight deck.
The armament consisted of four 0.30-inch (7.62 millimeters) 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 (13 millimeters) 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 (1633 kilos) of bombs.
The NA-62 was submitted in time to meet the Army's 10th of September deadline for the medium bomber competition. The Army was sufficiently impressed with the North American proposal that on 10th August, 1939, they issued a contract for 184 NA-62s under the designation B-25. This contract was finally approved on 10th September. 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 mock-up was approved on 9th November, 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 19th August, 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 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 (A separate streamlined enclosure on an aircraft for
sheltering or housing an engine) 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 geometry's 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 shape was attempted before the final, back-tilted fin and rudder configuration was adopted.
The idea of honouring 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 on this occasion.
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 8th January,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 aeroplane. 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 aeroplane 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.
In 1941, as a result of air combat reports coming in from Europe, modifications were introduced into the North American B-25 production line at Inglewood beginning with the 25th aircraft built, resulting in a change in designation to B-25A.
The B-25A introduced armour protection for the crew, including a 3/8-inch armour (10 millimeters) plate added to the pilot's, co-pilot's and bombardier's seats as well as to the gunner's compartments. The aircraft was also equipped with self-sealing fuel tanks, which reduced total fuel capacity from 912 to 694 US gallons (760 to 578 Imperial Gallons or 3454 to 2627 liters), although provision was made for the installation of a 418 gallon tank (1582 liters) in the bomb bay for ferrying. This made for a significant increase in weight, resulting in a slight degradation in the performance.
The first Air Corps operational unit with the B-25A was the 17th Bombardment Group at McChord Field, which included the 34th, 37th, and 95th Squadrons, plus the attached 89th Reconnaissance Squadron. This outfit, which formerly flew Douglas B-18s, moved to Pendleton, Oregon in June of 1941. Other B-25As were sent to the 30th Bombardment Group at New Orleans, the 43rd Bombardment Group at Bangor, Maine, the 39th Bombardment Group at Spokane, Washington, and the 44th Bombardment Group at MacDill Field in Florida. One B-25A went to Wright Field for tests.
A total of 40 B-25As were built before the production line switched over to the B-25B version.
Other armament changes introduced by the B-25B were less apparent. The stowable 0.30-inch (7.62 millimeters) waist gun of the B-25/B-25A was eliminated, and the side hatches with their Plexiglas windows were considerably reduced in size. The B-25B retained the 0.30-inch machine gun in the extreme nose that was operated by the bombardier. However, the 0.50-inch (13 millimeters) tail gun was eliminated and the position where the tail gun had been located was greatly reduced in size, and became little more than a prone observation post, being terminated at its extreme end with a transparent cap. The armour plate in the extreme tail was removed.
The crew was five -- pilot, co-pilot, bombardier/nose gunner, navigator/upper turret gunner, and radio operator/belly turret gunner. The length was reduced from 54 feet 1 inch (16.5 meters) on the B-25A to 52 feet 11 inches (16.2 meters) on the B-25B. The additional defensive armament of the B-25B caused the weight to creep up to 20,000 pounds (9072 kilos) empty and 28,460 pounds (12,910 kilos) fully loaded, resulting in another degradation in performance. Maximum speed was now down to 300 mph (482 km/h) at 15,000 feet (4572 meters).
The first 14 B-25Bs were accepted in August of 1941. The 17th Bombardment Group was the first to get the B-25B. By the time of Pearl Harbor on 7th of December 1941, North American had delivered 130 bombers. With the completion of the last of 120 B-25Bs in January of 1942, North American met the initial contract for 184 NA-62 aircraft.
The B-25J (NA-108) was the final production version of the Mitchell. It was also the version of the Mitchell to be built in the largest numbers, a total of 4318 being built. It was manufactured exclusively at North American's Kansas City plant, the Inglewood plant having switched over to the manufacture of the P-51 Mustang fighter after the last B-25H had been delivered.
Kansas City briefly built both the B-25D and J at the same time, the first J being accepted in December 1943 and the last D in March of 1944.
The B-25J returned to its primary function as medium bomber, and reverted to the transparent, bombardier-equipped nose of the earlier B-25C and D. The tail gun position with the deeper rear fuselage, the bay-window mounted waist guns, and the forward-mounted dorsal turret that had been introduced on the B-25H were all retained on the B-25J. The blister gun packs on the sides of the forward fuselage of the later versions of the B-25H were also retained. However, the co-pilot position (which had been omitted on the B-25H) was restored. The crew was now six -- pilot, co-pilot, navigator/bombardier/gunner, turret gunner/engineer, radio operator/waist gunner, and tail gunner. The bomb racks and bomb bay doors were now all electrically-operated. A 50,000 BTU surface combustion heater was provided at the waist gun station. Provision was made to carry three 1000 pound bombs (454 kilos) rather than just two. Alternatively, two 1600-pound (726 kilo) armour-piercing bombs could be carried. Provisions were made for the carrying of six 325-pound depth charges (147.4 kilos) on underwing racks.
The first B-25J (43-3780) took off on its first flight in October 1943, piloted by Joe Barton. The first USAAF acceptance took place before the end of the year.
The B-25J was built in eight main production blocks (-1, -5, -10, -15, -20, -25, -30, -35), with different suffix numbers being allocated to significant modifications, including -11, -17, -22, -27, -32, and -37. Many of these modifications involved the replacement of the transparent nose by a solid, eight-gun nose.
Beginning with the 151st B-25J-1 (43-4019), provisions for the carrying of a single 2000-pound bomb (907.2 kilos) were deleted. As it turned out, the 2000-pound bomb was only rarely carried during actual combat, and the bulky and restrictive shackles for the 2000-pound bomb took up a lot of space in the bomb bay. This enabled a normal offensive load of two 1600-lb (726 kilos)or three 1000-lb bombs to be carried internally, plus combinations of smaller bombs of various types, including 20-pound (9.07 kilos) parafrags.
The -5 production block introduced a revised braking system control cable. The N-3C gunsight replaced the N-3B sight and A-1 bombing head. De-icing windshield panels were installed, and gun-blast arrestors were installed on top turret guns and on side fuselage blister guns.
The -10 production block introduced the mounting lugs and controls for underwing bombs. Electric bomb racks were provided. The heaters at the waist gun positions were found to be inefficient and were removed.
The -15 production block had N-8A optical gunsights installed on the flexible waist guns. Provisions for ring and bead sights were provided for the flexible nose gun.
The -20 production block introduced some revisions to the cabin heating system with a 50,000 BTU/hour heater. A second 0.50-inch fixed machine gun was installed in the nose. The flexible nose gun was relocated 4 inches higher. Additional armour protection was provided in the floor of the nose for the bombardier. The top turret canopy was reinforced for greater strength, and a hydraulic emergency brake system was incorporated. Beginning with 44-29304, a change was made to the Holley 1685RB carburettor.
The -25 production block introduced new types of armoured seats for both pilots. Beginning with 44-30111, armoured plate deflectors were added to the upper fuselage to prevent the upper turret gunner from inadvertently firing his guns into the structure of his own plane, especially into the raised cupola where the tail gunner sat. Beginning with 44-30309 and throughout the -25 production block, provisions were made for the mounting of a chemical tank on an underwing bomb rack. On production block -30, stainless steel "S"-shaped exhaust stacks replaced the enamelled 1020 steel stacks on cylinders 1, 7, and 9. Effective with serial number 44-31111, provisions were made for the mounting of a chemical tank on an underwing bomb rack. Provisions for a type C-6 electric bomb hoist were made effective with 44-31311. Provisions were made for the carrying of wing-mounted T-64 zero-length rocket launchers beginning with 44-31338. These launchers could carry up to eight five-inch (127 millimeters) high-velocity aircraft rockets (HVAR). Beginning with 44-31491, a K-10 computing gunsight was provided for the gunner in the tail turret, and M-8A gun mounts were provided for the tail guns. Provisions for the mounting of glide bombs suspended underneath the fuselage were added beginning with 44-86692. In addition, a special cockpit sight and release controls for the glide bomb were provided. An N-9B bombsight was installed beginning with 44-86793. Beginning with 44-86799, the rudder control cables were re-routed.
The -35 production block introduced provisions for the carrying and laying of aerial mines.
Some of the B-25Js ended up with training units, but most were issued to units in action in the Southwest Pacific. The first B-25Js arrived at Townsville and Nadzab depots in the summer of 1944. They were issued to the 38th Bombardment Group. The 345th BG received its B-25Js in September. Despite volume production, it was hard to meet the demand, and the 42nd Bombardment Group did not get its B-25Js to replace its ageing C and D models until late 1944.
In the Mediterranean theatre, the B-25J was issued to operational bomb groups on an as-required basis. In April 1944, the 310th Bombardment Group based on Corsica received its first B-25Js. The remaining groups in the 57th Bombardment Wing of the 12th Air Force transitioned to the B-25J throughout the remainder of 1944.
The US Marine Corps ordered 255 B-25Js under the designation PBJ-1J.
The transparent nose with its bombardier could be replaced at the factory by a solid nose that was equipped with four 0.50-inch machine guns. With this modification, the aircraft was designated as B-25J-11, -17, -22, -27, -32, or -37, depending on which production block the modification took place. With its maximum armament of eighteen guns, the solid-nosed B-25J was the most heavily-armed attack aircraft in the Allied arsenal. Sometimes, however, the package guns on the sides of the fuselage were deleted, the remaining fourteen guns being more than enough.
The last B-25J was delivered to the USAAF in August of 1945. The day after the war in the Pacific ended, the Kansas City plant was closed.
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. It required 8,500 original drawings and 195,000 engineering man-hours to produce the first one, but 9816 were produced from late 1939, when the contract was awarded to North American Aviation, through 1945, 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, Brazil and China.
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 1920's who had been the assistant Chief of the Army Air Service. He was court-martialled 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 Brigadier General.
A long with its counterparts, the Martin B-26 Marauder and the Douglas A-26 Invader, the B-25 performed up to and beyond its original design expectations. During the war, B-25s served in both the European and Pacific Theaters of Operations flying a wide variety of missions, including medium and low altitude bombing overland and anti-ship operations at sea. Very heavily armed for both defensive and offensive purposes, the B-25 could be fitted with as many as 18 heavy .50 caliber machineguns or a 75mm cannon and up to 14 machineguns, thus turning the bomber into a very formidable gun platform for strafing missions against shipping and ground targets, including strong fortifications and armoured units.
During WWII, North American Aviation produced just under 10,000 Mitchell bombers in a total of 10 major variants at plants in Inglewood, California and Kansas City, Kansas. In addition to service with the U.S. Army Air Force, those bombers were also used to good effect by the U.S. Navy and Marine Corps and the air forces of 17 foreign countries. The last operational B-25 was finally retired from the U.S. Air Force inventory in January 1959.
B-25J Mitchell 'Photo Fanny'
Carrying the serial number 44-30423, The Air Museum "Planes of Fame" North American B-25J Mitchell was built in Kansas City in 1944.
It served with the U.S. Air Force until the late 195Os when it was put up for disposal at Davis-Monthan AFB, Arizona. Purchased by a private owner from Sacramento, California, the Mitchell was finally obtained by The Air Museum in the early 1960s and has been kept in flying condition since then. It regularly appears at airshows throughout the southwest and is frequently used for movie and television projects, both as a camera platform and as a subject for various photo projects.
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