Early in this period the US started to seriously consider war preparations and later fought WW-II. Navigators, observers and bombardiers by the tens-of-thousands were needed. December 1941,WW-II was on for the USA. Aircraft type and production rates and pilot numbers determined the number of navigators, bombardiers and gunners to be trained. The Army Air Corps (AAC), renamed the Army Air Force (AAF) June 20, 1941 trained during WW-II over 45,000 navigators and 50,000 bombardiers. The greatest number of planes, cargo and bombers, required navigators. Radar navigation, radar bombardment and electronic aided navigation became universally utilized. The AAF developed some 18 airplanes that went in to production and many more test prototype planes during this period.

Navigator developments in this period:

• Large scale training of flight navigators started in 1939 in England at the Manston Navigation school. Ref History of Air Navigation by Arthur J. Hughes, Chapter III Historic Flights 1919-1930, published by George Allen and Unwin LTD, London England 1946. Page 73.
  
• Navigation school funded by the US Army for Aviation Cadets at Coral Gables via contract with Pan American Airways and the University of Miami, Florida produced the first trained navigators for the US Army in 1941 using Pan American Curtis Condors as navigator training aircraft. Approximately 45,000 navigators were trained in WW-II. Ref. Winged Shield, Winged Sword A History of the USAF Vol. I, by Bernard C. Nalty, Editor, Air Force History and Museums Program. Page 260.
  
• First averaging flight sextants produced for WW-II. Ref History of Air Navigation by Arthur J. Hughes, Chapter III Historic Flights 1919-1930, published by George Allen and Unwin LTD, London England 1946. Page 73.
  
• Bubble-horizon sextants, averaging sextants, gyro sextants, Plath periscopic sextants, Lepetit sextants, position-line computers, sphero-trigonometers, and astrographs developed, improved and employed by the US and English military. Ref History of Air Navigation by Arthur J. Hughes, Chapter VII Sextants, published by George Allen and Unwin LTD, London, England.
  
• Reflected direction finding (Radar) developed by the British was deployed first as the ground based Chain Home/Chain Home Low  50MHz, apx 200kW-800kW with pulse recurrence frequency of 50k/second with an oscilloscope display.
  
• Radar-guided night fighter interceptor aircraft, Bristol Beaufighters and US Douglas Bostons, were deployed in 1939-40 with secret airborne intercept radars Mark IV, and SCR-520 and the SCR-720. This led to the development of the Northrop P-61 night fighter interceptor with a its radar-interceptor-operator (RIO) crewmember and the APG-15 gun control radar. The RIO is today know as the Weapons Systems Operator (WSO). The WSO s duties remain to this day in modern fighter interceptor aircraft like the F-15E, as they were. Ref. www.en.wikipedia.org/wik/History_of_radar , and Air Force Museum Foundation, Inc. Friends Journal Vol.29, NO.4 Winter 2006/2007 .
  
• Early US Army Air Corps radar (which came from the British to begin with via secret agreement) led to the use of the creation of the British H2S/ASV bombing radar and the creation of the APQ-7/TS radar AN/APR-1 search receiver, AN/APR-2 recording search receiver, AN/APA-11 pulse analyzer and the B-29 employed AN/APQ-13Mickey X-band with XCR-667 radar manufactured by Bell Telephone Labs and MIT. These airborne radars required a radar operator (RO) The radar's display was a circular cathode ray scope called a plan position indicator (PPI) which showed the terrain below and around the aircraft. Ref. Bombardier A History by Volume II by Turner publishing Company and Mark A. Thompson, Radar Bombing Radar Bombing by F. H. Klopper. Ref.The Bombers of WW-II by William N. Hess, published by Lowe and B. Hould, Ann Arbor, MI. 1993/1994/1998. Pages 205, 246/247, 301/302 and A Genesis Workshop by Squire L. Brown, HO/ASC AFMC, W-P AFB, Ohio 2003. Page 34.
  
• Loran A (long-range navigation) low frequency, 2MHz pulsed time referenced-differential-delay hyperbolic signal LF/VLF AN/APN-4 and AN/APN 9 were deployed with special maps, as transoceanic navigation and bombing aids. Loran A systems required long wire aircraft trailing antenna, a reel in/out type of some 250 feet. There e were 47 station pairs in operation at the end of WW-II. Ref. The Bombers of WW-II by William N. Hess, published by Lowe and B. Hould, Ann Arbor, MI. 1993/1994/1998. Pages 301 and 303 and Aviation History by Anne Millbrooke, Published by Jeppesen-Sanderson 1999, Pages 7-34 and 7-35 on Loran. Avionics Navigation Systems by Myron Kayton and Walter Fried, Published by John Wiley and Son, NY, NY. Page 180.
  
• AT-7 followed by the Beech AT-11 were aircraft specifically built and modified for navigator and bombardier training. The AT-11 was configured with a transparent nose for the bombardier and his bomb sight (Norden and/or Sperry integrated with the C-1 autopilot) with additional stations in the aft fuselage area, bomb bay with internal bomb racks with Type B-7 shackles with A-2 releases and a top plexiglass turret. Over 40,000 navigators and 50,000 bombardiers (apx 90%) for WW-II were trained in these planes. Ref. Splendid Vision,Unswerving Purpose: by HO/ASC AFMC, AFHMP, USAF, W-P AFB, Ohio. Pages 188 and 189, and the AT-11 T.O. No. 01-90KC-2 AT-11 Technical Order by the AAF circa 1943 and web site: www.twinbeech.com , Sept 2006.
  
• AT-18, AT-38, B-18, B-34 and C-60s aircraft were also used as navigator and bombardier trainers. The navigator crews used B-2/3/5 driftmeters, astrodomes for the A-10, A-12, MA-2 and AN 5851 Bubble Sextant types (hung by sky-hooks) bubble-averager-sextants and gyrostabilized compass systems. The Bombardiers trained on D-8, Sperry and Norden bombsights and dropped the 100 pound M38A2 high explosive bombs. Ref. Training to Fly: Military Flight Training by Rebecca (Hancock Cameron) Welch of the Air Force History and Museums Program, 1999, Chapter 12 Not Just a Pilot's War: Individual Training of Navigators, Bombardiers and Gunners. Pages 421-450.
  
• Air Intercept (AI) radar (SCR-720, built by MIT Radiation Laboratory) was developed for use by the Radar Observers Intercept Officers (RIOs) on the Northrop P-61 Black Widow. These RIOs would later crew the F-82, F-89 and F-94C. Ref. A Genesis Workshop by Squire L. Brown, HO/ASC AFMC, W-P AFB, Ohio 2003. Pages 34 and 35.