The Place Promised in Our Early Days (2004)

Set in an alternate ending to World War II, when the islands of Japan are divided into Union-controlled North and the US-controlled South, and the island of Hokkaido is annexed. A tall tower was built on Hokkaido, which could even be seen from Tokyo. In the summer of 1996, three middle-school students make a promise that they'll cross the border with a self-constructed plane and unravel the tower's secret...