Funding started in the 1960s, while the US Air Force searched for a replacement for the ageing EC-121 AWACS aircraft, with a 1963 requirement for up to 64 aircraft.[N 1] Boeing was chosen to construct the two airframes designated EC-137D, one for Westinghouse Electric´s and another one for Hughes´s radar. Westinghouse´s design was chosen as the winner. Full scale development began on 26 January 1973. Testing phase on the first E-3 Sentry began in 1975.
- E-3A: NATO standard aircraft, developed as an improved version of the USAF Core E-3A.
- E-3B: First 24 USAF E-3A aircraft upgraded with CC-2 computer also used on NATO aircraft.
- E-3C: Later USAF aircraft with improved command/control capability.
- E-3D: RAF version with Lorel Yellow Gate ESM wingtip pods and CFM-56 turbofans. Designated Sentry AEW Mk 1 in RAF service.
- E-3F: French variant.
- This was later reduced on economic grounds.
- World Aircraft Information Files Aviation Partwork. Midsummer Books Ltd. File 890 Sheet 62 (A-Z of Aircraft:B - Boeing Model 707 (continued) to Boring Model 707 (E-3 Sentry, E-6 Mercury and E-8 J-STARS)
- World Aircraft and Systems Directory - Third Edition. 2002. Reed Business Information Ltd. ISBN 0 617 01289 X Page 362
- WA&SD - Third edition. Page 363