B-25-NA

History

By 1938, the threatening situation in Europe had grabbed the attention of the United States Army Air Corps. The light bombers of the time were deemed unsatisfactory and new specifications were considered. At this point in history, there was no real meaning for the designation of "medium bomber". Increases in technology were increasing the options available. On March 11, 1939, the Air Corps Proposal number 39-640 was issued. The proposal specified a bomb load of 3,000 pounds, a range of 2,000 miles, and a top speed in excess of 300 mph. The new bomber would operate at altitudes from 8,000 to 14,000 feet.

The proposal was revolutionary in the day. The contracts would be issued based on the bidder's specification, drawings and performance. This was risky as there was no guarantee that the aircraft could be built to the manufacturer's specifications on time. The payoff would be in time. It was not uncommon for a manufacturer to test an experimental aircraft for two to three years before a contract would be awarded. The United States did not have that kind of development time.

The B-25 was North American's proposal to meet that requirement. There were three other aircraft up for consideration. The Martin B-26, Douglas B-23, and the Stearman P-23. The B-25 offered an aircraft easy to build, fly and maintain. The overall cost of the B-25 would be significantly less than the Martin B-26. In the end, the Army would award contracts to both the B-25 and the B-26.

Lee Atwood, North American vice president and chief engineer, was charged with the medium bomber project. His goal was to create an aircraft that was easy to repair and maintain as well as easy to fly. Time has proven his results. North American had just learned some good lessons from the NA-40 test program. The basic design of the B-25 would utilize many of the features seen in the NA-40. Tricycle landing gear, twin vertical tails with similar shape, root airfoil, identical engines and cowling shapes, constant dihedral and underhung nacelles were all drawn from the NA-40. The B-25 would also include the Hamilton propellers that were planned but never installed on the NA-40. But, the B-25 would not be another version of the NA-40. Side by side pilot seating, greater speed, range and payload capacity differentiated the two aircraft. The wing was moved to a shoulder position and the gross weight was increased by 8,000 pounds. North American preferred the Pratt & Whitney R-2800 engine, an unproven power plant at the time. With approximately 300 HP more than the Wright R-2600 engines, they also had a slightly smaller outside diameter which translated into less drag. In the end however, the proven Wright R-2600's would be specified.

On September 5, 1939, the North American General Order NA-62 was issued and construction of the first airplane commenced. The B-25 would have an outside width of only 56.5 inches. Broader than the NA-40, but this allowed comfortable side by side pilot seating and increased bomb bay volume. Armament was minimal with a .30 caliber flexible machine gun mounted in the nose that was movable to any of three locations. Another .30 caliber gun served as a waist gun in the rear fuselage. The rear received a .50 caliber gun mounted in an odd "clamshell" plexiglass canopy. This configuration gave the weapon a superb field of fire. Manufacturing of the B-25 started in North American's Inglewood, California plant. The B-25, B-25A, B-25B, B-25C, B-25G, and the B-25H were exclusively built at that plant. The California plant also sent the parts for the first 100 planes to be manufactured at the newly built North American Kansas plant at the Fairfax airport in Kansas City, Kansas.

On August 19, 1940, the first B-25-NA took to the air piloted by, test pilot Vance Breese and test engineer, Roy Ferren. After the first flight, engineer Ferren reported a severe roll-yaw condition existed. The B-25 test program continued with few problems. The nose gear collapsed due to a shimmy damper problem during taxi tests. On one test flight, a fuel line ruptured that damaged the aircraft and forced a wheels up landing on the field between the runways at Mines Field. There were no severe injuries in the incident and the aircraft was quickly repaired. During testing, there were five vertical tail configurations evaluated, the fifth being the current familiar design. The twin tails were seen to provide maximum control with a smaller profile and therefore harder to attack from the rear. Captain Frank Cook flew the plane for the Air Corps to test general handling characteristics. He determined the plane had a "Dutch roll" characteristic that he considered incompatible with bomb runs. The solution to this problem was to negate the dihedral of the outer wing panels. This gave the B-25 it's current "gull wing" configuration. The current wing shape was made effective on the 10th aircraft. There is some anecdotal information that the first 9 aircraft were modified. Aircraft 40-2166, 40-2170, 40-2173, 40-2174 and 40-2176 were briefly returned to Inglewood from their first assigned unit. There is no indication as to the reason for this recall. Aircraft 40-2168 was indeed modified as it has the current "gull wing" configuration. This likely occurred in 1943 when the aircraft was modified by North American for use as General Arnold's personal transport. In my research, I came across this photo that clearly shows one of the first 9 B-25-NA bombers along with later B-25-NA bombers. This would tend to confirm that at least one of the B-25-NA bombers built with a constant dihedral wing was not immediately modified.

Although the B-25-NA performed better than expected, there were some limitations. Due to the lack of armor protection and self-sealing fuel tanks, the B-25-NA's that were produced were destined for training or transports. The majority of the aircraft (19) were sent to McCord Field to train with the 17th BG.

• Model: B-25-NA
• Total Produced: 24
• Manufacturing Plant: Inglewood, California
• First Test Flight: August 19, 1940
• First Flight Crew: Pilot: Vance Breese, Engineer: Roy Ferren
• Engines: Wright R-2600-9
• Carburetors: Bendix Stromberg PD-13E-2
• Fuel Capacity:
  • 2 forward wing tanks, total 484 gallons
  • 2 rear wing tanks, total 432 gallons
  • Droppable bomb bay tank 420 gallons
• Armament:
  • 3 flexible .30 caliber guns in nose, waist and tail
  • 1 flexible .50 caliber gun in tail
• Armor Protection: None
• Weights:
  • Empty: 17,258 lbs.
  • Max: 28,557 lbs.
• Speed (Max): 322 mph at 15,000 feet
• Service Ceiling: 30,000 ft
• Range: 2,000 miles with 3,000 lbs. bombs
• Crew:
  • Pilot, co-pilot, bombardier, navigator/radio operator, gunner
• Production by year:
  • 1941: 24
• First Airplane Accepted: February 17, 1941
• Last Airplane Accepted: May 6, 1941

• NAA Contract:
  • NA-62 signed September 5, 1939
• Major Visual Production Block Changes:
  • Single production block
• Notable Aircraft:
  • 40-2165 the first B-25 built.
  • 40-2168 the only surviving aircraft from this production block, and the fourth B-25 built.