Kumo no Mukou, Yakusoku no Basho The Place Promised in Our Early Days
Set in an alternate universe, Japan has been divided since World War II. To the north, Hokkaido has been annexed by the "Union" while Honshu and other southern islands exist under US sovereignty. A gigantic and mysterious tower has risen in the north, easily viewed across the Tsugaru Strait from Aomori, the northernmost prefecture of Honshu.
In the summer of 1996, three 9th-graders had made a promise that one day they'll build an aircraft and unravel the tower's mystery, but their project was abandoned after the girl, Sayuri Sawatari, began experiencing sleeping sickness and transferred to Tokyo for better treatment. Three years later, Hiroki Fujisawa has accidentally found out that Sayuri has been in coma since that time and with the help of his friend, Takuya Shirakawa, he will seek a way to revive her.
What the friends don't know is that Sayuri's unconsciousness is somehow linked with secrets of the tower... and the world.