The four-engine plane carried 15 persons, nine crew and six passengers and nary a trace was ever found. One passenger, a Chinese-American (Chinese were not permitted to be citizens then), was reported to be carrying $US3 million for use by the Chinese government in its war with Japan.
One modern researcher, Guy Noffsinger, has started a website focusing on the mystery of the Hawaii Clipper and is investigating the theory that the flight was hijacked by Japanese Naval intelligence agents to Truk (modern day Chuuk) where the passengers and crew were murdered and buried. Noffsinger has visited Chuuk several times and tried to locate the burial site based on second- and third- hand accounts from natives hired to inter the 15 bodies. There are many allegations, but little in the way of evidence.
There is the theory that the Japanese wanted the money or wanted the new engines from the airplane or wanted to ransom the passengers. These are just theories without any evidence.
Stronger, in my opinion are two comments – included on the website – from contemporary witnesses to the effect that the pilot was not particularly talented and that the Martin M-130 flying boat, was fundamentally unstable. The last position report has the plane flying just a few hundred feet below the cloud cover far over the Pacific. In those days, instrument flying was in its infancy and it remains easy for a pilot to experience a “loss of situational awareness” and inadvertently enter a spin. This is how John F. Kennedy, Jr., and his two passengers died over Nantucket Island. He got into the clouds and entered a spin.
None of this helps the MH370 question, but we are reminded that flying over water can be fundamentally hazardous.