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, and the mainwheels 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 as N2825B.
40-2165/2188 North American B-25 Mitchell company numbers 62-2834/62-2857
Engines: 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.