In April 1974 (1974-04), the carrier placed an order for two Airbus A300s, one A300B2 and one A300B4, taking an option for one more of the longer-range variant. After taking delivery of the first of these aircraft on a lease basis from Airbus in October 1974 (1974-10), Air Siam became the second operator worldwide in deploying the type on scheduled services, after Air France.:590 Also in 1974, the company placed an order for a Douglas DC-10-30, to be used on the Bangkok–Los Angeles route. Rights for the routes the airline intended to deploy the A300s on were never granted, and the A300B2 was returned to the lessor just a year after it was incorporated into the fleet.
Before Air Siam started flying long-haul services, Thailand's flag carrierThai International was the country's single long-haul operator. Already in 1974, the airline had accused both Thai International and the Thai Civil Aeronautics Board for obstructing their expansion plans. Despite various governments had considered the possibility of a merger, this never materialised. The new government that took office in 1976 had a different vision, and was decided for Thailand to have just one long-haul carrier.:1564
^"Air Transport...". Flight International: 919. 24 June 1971. Archived from the original on 19 October 2012. Retrieved 19 October 2012. "Air Siam reports excellent load factors for its first two months' trans-Pacific operations to Los Angeles. Overall load factor on the twice-a-week passenger/freight service has been 50 per cent, much higher than budgeted, and increased frequency is being considered. Air Siam operates a DC-8-63 wet-leased from Overseas National."
^"Air Transport...". Flight International: 444. 1 April 1971. Archived from the original on 19 October 2012. Retrieved 19 October 2012. "Air Siam is reported to be ready after long delays to begin trans-Pacific services with a DC-8 wet-leased from ONA. TWA is rendering operational assistance to Air Siam."
^"World airlines update". Flight International: 513. 17 October 1974. Archived from the original on 30 November 2012. Retrieved 30 November 2012. "Air Siam has leased an A300B2 from Airbus Industrie pending the delivery of a new A300B4 next year. The aircraft will replace 707s on the Bangkok-Hong Kong route. Air Siam has also taken an option on a further A300B4 and ordered a DC-10-30 for use between Bangkok and Los Angeles. Additionally, the airline has extended its lease of a single 747 from Aer Lingus until May 1976."
"Airliner prices index (page 590)". Flight International: 590–591. 31 October 1974. Archived from the original on 19 October 2012. Retrieved 19 October 2012. "The European A300 is now in service with two airlines. Air France began operations on April 22 between London and Paris and Air Siam took delivery of a leased A300B2 last week."
^"Siamese DC-10". Flight International: 291. 12 September 1974. Archived from the original on 30 November 2012. Retrieved 30 November 2012. "Air Siam has ordered a single DC-10-30 for delivery later this year. It will be used on services between Bangkok and Los Angeles, which are due to start on November 25."
^"Airliner market". Flight International: 648. 30 October 1975. Archived from the original on 30 November 2012. Retrieved 30 November 2012. "Air Siam is returning its leased A300B2 to Airbus Industrie on the expiry of the lease today. The reason given was that the routes which the airline had in mind when it took on the aircraft have not been granted; the airline has had more success on its longer routes, for which the A300B2 lacks range. Air Siam intends to acquire a second McDonnell Douglas DC-10."
^"Air Transport". Flight International. 27 June 1974. Archived from the original on 19 October 2012. Retrieved 19 October 2012. "Air Siam has accused Thai International and the Thai Civil Aeronautics Board of obstructing its plans for expansion."