A picture that has never before been revealed proves the Japanese army bombed Yan'an, which is known as the "red cradle" of the Chinese revolution, during World War II.

It has been questioned why the Japanese only bombed Chongqing, leaving Yan'an untouched, during the war. "Obviously the doubts are ungrounded," said Feng Yan, curator of the Beibei District Archives in Southwest China's Chongqing municipality, on Tuesday.

Feng revealed a picture, which had never been shown before, depicting the Japanese army bombing Yan'an. In addition, multiple "valuable historical materials are to be seen for the first time" to show the Chinese Communist Party's efforts against Japanese invaders during the war, it was added.

Beibei District Archives have been collecting Anti-Japanese War materials for three years, said Feng. Now they have hundreds of book records, medals, certificates of honor and construction records from the period.

Without a large population or major strategic targets, Yan'an was bombed far less than Chongqing.

However, on October 15, 1939 the Japanese Army unleashed 71 flights in four groups to bomb the city. At 9 a.m., 36 Japanese bombers dropped more than 100 bombs. At 2 p.m., another 35 bombers in three rotations took turns to drop a combined 120 bombs. The air raid killed 10 and injured 13 while a large number of houses were set on fire.

According to an undated Asashi News report, multiple Japanese troops bombed Yan'an at 11.30am to hit various targets, including military facilities and schools. Smoke covered the city as the Japanese continued to bomb during the afternoon.