Although certified as Series 100s, 200s, 300s, etc., the different 707 variants are more commonly known as Series 120s, 220s, 320s, and so on, where the "20" part of the designation is
Boeing's "customer number" for its development aircraft.
707-020 Announced in July 1957 as a derivative for shorter flights from shorter runways, the 707-020 first flew on November 23, 1959. Its type certificate was issued on June 30, 1960, and it entered service with United Airlines on July 5, 1960. As a derivative, the 720 had low development costs, allowing profitability despite few sales. Compared to the 707-120, it has a length reduced by 9 feet (2.7 m), a modified wing and a lightened airframe for a lower
maximum takeoff weight. Powered by four Pratt & Whitney JT3C turbojets, the initial 720 could cover a range with 131 passengers in two classes. Powered by JT3D turbofans, the 720B first flew on October 6, 1960, and entered service in March 1961. It could seat 156 passengers in one class over a range. A total of 154 Boeing 720s and 720Bs were built until 1967. Some 720s were later converted to the 720B specification. The 720 was succeeded by the Boeing 727 trijet.
707-120 The 707-120 was the first production 707 variant, with a longer, wider fuselage, and greater wingspan than the Dash 80. The cabin had a full set of rectangular windows and could seat up to 189 passengers. It was designed for transcontinental routes, and often required a refueling stop when flying across the North Atlantic. It had four Pratt & Whitney JT3C-6 turbojets, civilian versions of the military J57, initially producing with
water injection. Maximum takeoff weight was and first flight was on December 20, 1957. Major orders were the launch order for 20 707-121 aircraft by Pan Am and an American Airlines order for 30 707-123 aircraft. The first revenue flight was on October 26, 1958; 56 were built, plus seven short-bodied -138s; the last -120 was delivered to Western in May 1960.
707-138 The 707-138 featured a -120 fuselage, from which (three frames) were removed both ahead of and behind the wing, increasing range. The maximum takeoff weight remained the same as the standard version, at . The variant was produced for Qantas and included their customer number, 38, in its designation. To allow for full-load takeoffs at the midflight refueling stop in Fiji, the wing's leading-edge slats were modified for increased lift, and the allowable temperature range for use of full takeoff power was increased by 10 °F (5.5 °C). Seven -138s were delivered to Qantas between June and September 1959, and they first carried passengers in July of that year.
707-120B The 707-120B had Pratt & Whitney JT3D-1 turbofan engines, which were quieter, more powerful, and more fuel-efficient, rated at , with the later JT3D-3 version giving . (This thrust did not require water injection, eliminating both the system and 5000–6000 lb of water.) The -120B had the wing modifications introduced on the 720 and a longer tailplane; a total of 72 were built, 31 for American and 41 for TWA, plus six short-bodied -138Bs for Qantas. American had its 23 surviving -123s converted to -123Bs, but TWA did not convert its 15 -131s. The only other conversions were Pan Am's five surviving -121s and one surviving -139, the three aircraft delivered to the USAF as -153s and the seven short-bodied Qantas -138s (making 13 total 707s delivered to Qantas between 1959 and 1964). The first flight of the -120B was on June 22, 1960, and American carried the first passengers in March 1961; the last delivery was to American in April 1969. Maximum weight was for both the long- and short-bodied versions.
707-220 The 707-220 was designed for
hot and high operations with more powerful Pratt & Whitney JT4A-3 turbojets. Five of these were produced, but only four were ultimately delivered, with one being lost during a test flight. All were for Braniff International Airways and carried the model number 707-227; the first entered service in December 1959. This version was made obsolete by the arrival of the turbofan-powered 707-120B.
707-320 Intercontinental turbojets The 707-320 Intercontinental is a stretched version of the turbojet-powered 707-120, initially powered by JT4A-3 or JT4A-5 turbojets producing each (most eventually got JT4A-11s). The interior allowed up to 189 passengers, the same as the -120 and -220 series, but improved two-class capacity due to an 80-in fuselage stretch ahead of the wing (from to ), with extensions to the fin and horizontal stabilizer extending the aircraft's length further. The first announced customer was
Lufthansa.
BOAC's controversial order was announced six months later, but the British carrier got the first service-ready aircraft off the production line. The British
Air Registration Board refused to give the aircraft a certificate of airworthiness, citing insufficient yaw control, excessive rudder forces, and the ability to over-rotate on takeoff, stalling the wing on the ground (a fault of the de Havilland Comet 1). Boeing responded by adding to the vertical stabilizer, applying full instead of partial rudder boost, and fitting an underfin to prevent over-rotation. These modifications except to the fin under the tail became standard on all 707 variants and were retrofitted to all earlier 707s. The 37 -420s were delivered to BOAC, Lufthansa,
Air-India,
El Al, and
Varig through November 1963; Lufthansa was the first to carry passengers, in March 1960.
707-320B turbofans The 707-320B had the application of the JT3D turbofan to the Intercontinental, but with aerodynamic refinements. The wing was modified from the -320 by adding a second inboard kink, a dog-toothed leading edge, and curved low-drag wingtips instead of the earlier blunt ones. These wingtips increased overall wingspan by 3.0 ft (0.9 m). Takeoff gross weight was increased to . The 175 707-320B aircraft were all new-build; no original -320 models were converted to fan engines in civilian use. First service was June 1962, with Pan Am. The 707-320B Advanced is an improved version of the -320B, adding the three-section leading-edge flaps already seen on the -320C. These reduced takeoff and landing speeds and altered the lift distribution of the wing, allowing the ventral fin found on earlier 707s to be removed. From 1965, -320Bs had the uprated -320C undercarriage allowing the same MTOW. These were often identified as 707-320BA-H.
707-320C passenger–freight configuration The 707-320C has a convertible passenger–freight configuration, which became the most widely produced variant of the 707. The 707-320C added a strengthened floor and a new cargo door to the -320B model. The wing was fitted with three-section leading-edge flaps which allowed the removal of the underfin. A total of 335 of this variant were built, including some with JT3D-7 engines ( takeoff thrust) and a takeoff weight of . Most -320Cs were delivered as passenger aircraft with airlines hoping the cargo door would increase second-hand values. The addition of two new emergency exits, one on each side aft of the wing, raised the maximum passenger limit to 219. Only a few aircraft were delivered as pure freighters. One of the final orders was by the Iranian Government for 14 707-3J9C aircraft capable of VIP transportation, communication, and in-flight refueling tasks.
707-700 The 707-700 was a test aircraft used to study the feasibility of using CFM International CFM56 engines on a 707 airframe and possibly retrofitting existing aircraft with the engine. After testing in 1979,
N707QT, the last commercial 707 airframe, was restored to 707-320C configuration and delivered to the Moroccan Air Force as a tanker aircraft via a "civilian" order. Boeing abandoned the retrofit program, since they felt it would be a threat to the 757 and 767 programs. The information gathered from testing led to the eventual retrofitting of CFM56 engines to the USAF C-135/KC-135R models, and some military versions of the 707 also used the CFM56. The Douglas DC-8 "Super 70" series with CFM56 engines was developed and extended the DC-8's life in a stricter
noise regulatory environment. As a result, significantly more DC-8s remained in service into the 21st century than 707s.
Undeveloped variants The 707-620 was a proposed domestic range-stretched variant of the 707-320B. The 707-620 was to carry around 200 passengers while retaining several aspects of the 707-320B. It would have been delivered around 1968 and would have also been Boeing's answer to the stretched
Douglas DC-8 Series 60. Had the 707-620 been built, it would have cost around US$8,000,000. However, engineers discovered that a longer fuselage and wing meant a painstaking redesign of the wing and landing-gear structures. Rather than spend money on upgrading the 707, engineer
Joe Sutter stated the company "decided spending money on the 707 wasn't worth it". The project was cancelled in 1966 in favor of the newer
Boeing 747. The 707-820 was a proposed intercontinental stretched variant of the 707-320B. This variant was to be powered by four Pratt & Whitney JT3D-15 turbofan engines, and it would have had a nearly extension in wingspan, to . Two variations were proposed, the 707-820(505) model and the 707-820(506) model. The 505 model would have had a fuselage longer than the 707-320B, for a total length of . This model would have carried 209 passengers in mixed-class configuration and 260 passengers in all-economy configuration. The 506 model would have had a fuselage longer than the 707-320B, to in length. This second model would have carried 225 passengers in mixed-class configuration and 279 passengers in all-economy configuration. Like the 707-620, the 707-820 was also set to compete with the stretched DC-8-60 Super Series models. The design was being pitched to American, TWA, BOAC, and Pan Am at the time of its proposal in early 1965. The 707-820 would have cost US$10,000,000. Boeing 717 was the company designation for the
C-135 Stratolifter and
KC-135 Stratotanker derivatives of the 367-80. (The 717 designation was later reused in renaming the McDonnell Douglas MD-95 to
Boeing 717 after the company merged with Boeing.)
Comparison of variants Below is a list of major differences between the 707 variants. ==Operators==