The Draken was developed by Saab through the 1950s to replaced the J 29 Tunnan and supplement the J 32 Lansen. Entering service in 1960 the aircraft uses a unique double delta wing to provide much better field performance than other classic deltas while still being capable of speeds in excess of Mach 2. Initial aircraft were the J 35A fighters powered by the RM6B (licence built RR Avon 300) engine. The J 35B featured the S7 fire control radar and the J 35D introduced the uprated RM6C engine. The final new build Swedish fighter version was the S 35 F. This featured a Hughes weapon control system comprising a pulse Doppler radar, automatic fire control system and Falcon AAMs. The J 35F-II sub-variant included a Hughes IR sensor. After the F model 66 were upgraded to F 35J standard and this variant served until 1999. The Swedish air force also operated reconnaissance S 35s and trainer Sk 35s. Denmark and Finland purchased the Draken but have now retired the aircraft. Austria is now the only operator, flying 24 rebuilt ex Swedish aircraft. These are expected to serve until approximately 2003/04 and were to be replaced by Eurofighter Typhoons but this project is now in doubt.