Kumo no Mukou, Yakusoku no Basho (Novel)

From Yen Press: In the Cold War era of an alternate history, the Soviet Union has taken control of the northern island of Japan and cut it off from the rest of the country. Just south across the strait, a boy named Hiroki is fascinated by the mysterious tower the Soviets have constructed on the unreachable island, and he and his friend Takuya decide to build a plane that will take them over to see it. As they work, a girl named Sayuri becomes a part of their lives and the promise to one day fly to the tower. But when she disappears without a trace, their promise I left unfulfilled—possibly forever.