The Pan American Airways (Pan Am) began its first venture crossing the Pacific route in 1935. The flight had a huge implication to subsequent airmail routes preceding to the WWII and many covers from Sarawak, Brunei and North Borneo were flown through it. Although more expensive than usual, covers brought by the flights are considered safer especially at the height of war.
On the 22nd November 1935, the Martin M-130 (known as China Clipper) under the command of Captain Edwin C. Musick left San Francisco for Honolulu, Midway, Wake, Guam and finally Manila. After a glorious welcome and a short stay in Manila, the return flight started on the 2nd of December and the flying boat arrived in San Francisco on the 6th of December 1935.
San Francisco – Honolulu – Midway – Wake – Guam – Manila – Maçao – Hong kong
On 21st April 1937, the route was extended to Hong Kong where it connected with China national Aviation Corporation service to Shanghai. Mail was accepted for Manila, Guam, Honolulu and san Francisco from Penang and Singapore. The mail would leave Penang by Imperial Airways for Hong Kong each Monday and from thence connect with the Pan Am service.
During this time period, letters from Sarawak and Brunei may be flown to Hong Kong by B.O.A.C flights for connection with the Pan Am service. Such covers are usually endorsed with “B.O.A.C & Pan American Airways” or similar endorsement to that effect.
San Francisco – Honolulu – Midway – Wake – Guam – Manila – Singapore
The Pan American Airways service was extended to Singapore on May 10th 1941. Up until the 1st October 1941, the clippers terminated on alternate weeks either at Singapore or at Hong Kong.
With the extension of the Pacific service to Singapore, there was a need for an additional aircraft to operate the shuttle service from Manila to Hong Kong. This was provided by a Sikorsky aircraft thereby eliminating the Boeing Clippers from this section.
This new route lead to the postal rates for destinations to the US and Europe being reduced.
With the attack of Pearl Harbour by the Japanese Imperial Army on the 7th December 1941 and the US officially declaring war against Japan, the Pacific routes service were immediately suspended.
Further reading: Trans-Pacific Clipper Service.