In the early 1960s, Sweden was looking for a multirole fighter that could replace the SAAB 32 Lansen in the strike role and the SAAB 35 Draken in the air defense and reconnaissance roles. The specifications for the new aircraft were ambitious:
It was to have Mach 1 speed at low altitude and Mach 2 speed at high altitude.

It was to act as an element in Sweden's STRIL 60 integrated air defense network.

It was to be suitable for operations in the Swedish BASE-90 scheme. Under BASE-90, sections of specially prepared roadway 800 meters long and 9 meters wide (2,625 x 30 feet) were built to act as airstrips, with these specially prepared road segments associated with the basic infrastructure for a field base. In time of war, aircraft and their support units would disperse to these sites to reduce their vulnerability to attack.

The BASE-90 requirement meant that the aircraft needed good short-field capability, which tends to run at cross purposes to high performance. Complicating this issue was the fact that Sweden's weather tends toward the icy and slick for a good part of the year, and of course it gets icier the farther north one goes.
SAAB had been considering various designs to follow the Draken since 1952. The company was able to submit a proposal to meet the specification, the "SAAB System 37", in February 1962. The design featured a canard configuration that offered better short-field operation than a traditional delta wing, and a powerful RM8 afterburning bypass turbojet engine with a thrust reverser to cut down landing roll. The SAAB 37 would feature an automated microwave instrument landing system (ILS) to help perform short landings, and would also be equipped with state-of-the-art avionics and combat systems. In addition, since Sweden was and remains strongly reliant on large numbers of reservists who would be called up in time of war, the SAAB 37 was designed to be easily maintained by ground crews with relatively limited training.
The Svenska Flygvapnet (Swedish Air Force) liked the proposal and gave the go-ahead on 28 September 1962 for full development of the "Viggen (Thunderbolt)", as the type was named, with details released to the public in December 1962. Incidentally, "Viggen" actually has a somewhat more specific meaning, being the clap of thunder produced by the thunder god Thor's hammer Mjoelnir, but "Thunderbolt" gives the desired impression in any case.
Several different lines of Viggen development were envisioned:
The "AJ 37" attack variant, with secondary air combat capability.

The "SF 37" dedicated reconnaissance variant.
The "SH 37" maritime reconnaissance and strike variant.
The "Sk 37" two-seat operational conversion trainer.
The "JA 37" interceptor variant, based on an improved airframe.

Six single-seat AJ 37 Viggen prototypes and one two-seat Sk 37 Viggen prototype were ordered in 1965, at about the same time a mockup of the aircraft was displayed to the media. The first single-seat prototype was rolled out on 24 November 1966 and performed its initial flight on 8 February 1967, with Erik Dahlstrom, SAAB's chief test pilot, at the controls. Dahlstrom stated that the Viggen handled as pleasantly as a sportsplane. Pilots would always like the Viggen's handling, particularly in comparison to that of the more challenging Draken, but the steep landings were tricky to master, and even Dahlstrom scraped the tail at least once.
The second prototype followed on 21 September 1967, and the third on 29 March 1968. The last of the six single-seat prototypes was in the air by April 1969, with this machine being very close to production AJ 37 specification. The initial prototype had been involved in a fatal "sheer dumb bad luck" accident on 31 May 1968, when Lennart Fryoe dropped a flight checklist while preparing to take off and accidentally triggered the ejection seat; his parachute didn't open fully and he was killed. The aircraft kept on rolling until it hit a drainage ditch and hangar, but it was repaired and returned to service. The fourth prototype was lost in an accident on 7 May 1969, with another prototype built to take its place. It is unclear if this prototype was an additional aircraft or part of the seven originally ordered.
A production order for 175 AJ 37 / SF 37 / SH 37 machines had already been placed on 5 April 1968. Initial flight of the first production AJ 37 was on 23 February 1971, with initial service deliveries to the Flygvapnet in June 1971.

SAAB AJ 37 VIGGEN Specifications:

   Spec                    Metric                      Imperial

Wingspan                10.60 meters         34 feet 9 inches
Wing area               46.0 sq_meters     495.16 sq_feet
Canard span             5.45 meters          17 feet 10 inches
Canard area             6.20 sq_meters      66.74 sq_feet
Length                    16.30 meters          53 feet 6 inches
Height                      5.80 meters          19 feet

Empty weight            11,800 kilograms    26,015 pounds
Normal weight           15,000 kilograms    33,070 pounds
MTO weight              20,500 kilograms    45,195 pounds

Max speed at altitude   2,125 KPH           1,320 MPH / 1,145 KT
Service ceiling         18,300 meters       60,000 feet
Takeoff run             400 meters               1,310 feet
Landing run            500 meters               1,640 feet
Combat radius     1,000 kilometers      620 MI / 540 NMI

Main armament for the anti-shipping role, a very important role, is the Saab 304 rocket-powered anti-ship missile. For ground attack, 135 mm Bofors M70 rockets (either 21 kg GP warhead with 3.7kg explosive or 20 kg AP fragmentation warhead with 5 kg explosive) in pods of six each, were together with the command controlled, smoke-less liquid fuel Saab 305 missile, the main weapons, with the 120 kg bombs and 30 mm podded guns used when appropriate.
The Saab 305/Rb 05 missile is now an all round weapon, as it is rather effective against slow, large aircraft and helicopters too. A TV version was contemplated as the Rb 05B, but it was cheaper to buy Mavericks instead.
It has always had a secondary fighter role, with Sidewinders and 30 mm cannon in pods.

Technical data

                                        AJ/SH     SF        SK        JA
Take off run:                       400 m     400 m     400 m     400 m
Landing run:                       450 m     450 m     450 m     450 m
Landing speed:                   220 km/h 220 km/h 220 km/h 220 km/h
Length:                                16.30 m   16.50 m   16.30 m   16.43 m
Span:                                  10.6 m    10.6 m    10.6 m    10.6 m
Height:                                  5.6 m     5.6 m     5.6 m     5.9 m
w folded fin:                          4.0 m     4.0 m     4.0 m     4.0 m
Engine thrust:                      6690 kp   6690 kp   6690 kp   7415 kp
w afterburner:                    11790 kp 11790 kp 11790 kp 13125 kp
Range:                                2000 km   2000 km             2000 km
Ferry range would be            2250 km, about 15-16% more with Jet A1 fuel.
Empty weight:                      9500 kg   9500 kg             9500 kg
Max payload:                       3600 kg   2500 kg             1700 kg
                                  (excluding external tank)
Max take off
Weight:                              18000 kg 17000 kg            18600 kg
Max speed                      (at 3600 kg load?)
Low altitude:                        Mach 1.1   Mach 1.1           Mach 1.2
high altitude:                       Mach 2+    Mach 2+            Mach 2+
Guaranteed speed
High altitude:                        Mach 1.7   Mach 1.7           Mach 1.8
Time from brake release to         10 km or Mach 1 at low altitude: 100 s
Max altitude:                        18000 km   18000 m            18000 m