The Estonian Air Force had more than 130 aircraft in the middle of the 1930s. Estonian engineers had previously designed and constructed several trainer and light fighter aircraft types, of which the most famous is the PTO-4 which entered service in 1938 as a military training aircraft. Before the Soviet occupation, Estonia had placed an order for 12 Supermarine Spitfire fighters and 2 Westland Lysander aircraft from Britain, and was also seeking to purchase 12 Messerschmitt Bf 109 fighters from Germany. Deliveries were interrupted by the start of World War II and the Spitfires later served in the Royal Air Force.