"In view of budgets and production schedules a new acquisition of around 25 planes is appropriate," said one of the sources with knowledge of the plan. The sources asked not to be identified because they are not authorized to speak to the media.
The sources said buying complete aircraft from the United States, at about $100 million each, will save Japan about $30 million per airframe.
The purchase will add to an earlier order for 42 of the fighters, most of which are being constructed at a "final assembly and check out" plant in Japan operated by Mitsubishi Heavy Industries, the country's leading defence contractor.
That plant is one of only two such factories outside the United States. The other, in Italy, is operated by Leonardo Spa.
As China fields ever more advanced aircraft, including stealth planes, and as North Korea pushes ahead with its nuclear weapon and ballistic missile programmes, adding F-35s will further increase Japan's reliance on US military technology to give it an edge over potential foes in East Asia.
Japanese military planners are also considering buying F-35Bs, the vertical takeoff and landing (VTOL) version of the aircraft. Those models can operate from small islands skirting the East China Sea or from ships such as the Izumo-class helicopter carriers.
"We have not yet made any plan and we are evaluating what fighter aircraft we need," Itsunori Onodera said at a news briefing on Tuesday when asked whether Japan planned to buy more F-35s.