A major change became evident on 15. June 1937 when Aero O/Y accepted its first landplane in the form of de Havilland DH-89 Dragon Rapide, and soon entered service inbetween Riga and Liepaja. By the time of the Russo-Finnish "Winter War" of 1939/40 Aero O/Y's fleet comprised two JU-52/3m and two DH-89 aircraft. The "Winter War" resulted in the cancellation of only eight of Aero O/Y's scheduled 389 flights, six of these being attributable to weather conditions rather than Soviet action, but during the "Continuation War" of 1941-44, when Finland was allied with Germany, all services were cancelled.

Aero O/Y resumed operations in August 1945 with domestic services from Hyvinkää to Vaasa, Kerni and Javäskylä, and in 1946 the government of Finland acquired a 70 % shareholding in the airline. Aero O/Y accepted the first of six Douglas DC-3 twin-engined transports on 6. June 1947, and resumed services to Stockholm on 1. November: the airline had earlier operated two examples of the Douglas DC-2 from 28. April 1941. The availability of the DC-3 fleet permitted an expansion of the airline's international network, and on 15. April 1951 Düsseldorf and Hamburg were added to the destinations served by the airline, which was now operating under the name Finnair although this change was not formally adopted until July 1968.

Finnair aquired its first truly modern aircraft on 27. January 1953, when it took delivery of its initial Convair CV-340, and with this type Finnair was able to start a service linking Helsinki with London, via Hamburg and Amsterdam, on 1. September 1954. Finnair also became the first non-Soviet bloc airline to serve Moscow with the start of a CV-340 service on 18. February 1956.

Finnair missed out the intermediate step of turboprop-powered airliners, and instead proceeded straight to the pure-jet type with a January 1958 order for three examples of the Sud-Aviation Caravelle IA: the first of the aircraft was accepted on 18. February 1960 and the type operated its first revenue-earning services on 1. April on the routes from Helsinki to Stockholm and Frankfurt. Finnair received the first of its larger turbofan-powered Super Caravelle 10B aircraft on 22. July 1964, and this type remained in service until 30. April 1983, when a final service was flown from Helsinki to Monastir in Yugoslavia.

With the Super Caravelle established as the workhorse of Finnair's international network, the CV-340 replaced the DC-3 on the airlines's domestic network. In general, however, Finnair was notably cautious in its expansion after World War II, and it was not until November 1966 that the airline placed its first order for long-haul aircraft in the form of two McDonnell Douglas DC-8-62CF four-turbofan transports with which to expand the international network to North America. Finnair accepted delivery of its first DC-8 on 27. January 1969, and this soon entered service between Helsinki and New York via two intermediate stops. Finnair was satisfied with the DC-8, and accordingly ordered the McDonnell Douglas DC-9-10 for its short-haul routes, accepting the first of these aircraft on 24. January 1971. Since that time Finnair has operated two other variants of the basic DC-9 series, as well as three variants of the lengthened and modernised McDonnell Douglas MD-80 series: the Finnish operator received its first examples of the DC-9-41, DC-9-51 and MD-82 in March 1981, January 1976 and March 1983 respectively.

Finnair received its first wide-body airliner, a McDonnell Douglas DC-10-30, on 27. January 1975, and in its original DC-10 and updated McDonnell Douglas MD-11 forms this three-turbofan transport replaced the DC-8-62CF on the North American and most intercontinental routes. In April 1980 Finnair bought the first two of an eventual three Fokker F-27 Friendship twin-turboprop transports from Icelandair for the operations of low-density services to destinations such as Umeå, Vaasa and Turku in the remoter parts of Finland.

The F-27 aircraft did not remain in service for long, and while it still operates mainly McDonnell Douglas aircraft, Finnair did also operate Airbus aircraft in the form of two A300B4 transports.