mid-1939, Japanese forces intensified their attacks on the front near Lanzhou. The Japanese air units were operating from airfields in Shanxi at the margin of the operating range of their fighters, and their bombers were often not escorted by fighter planes. On 20 February 1939, thirty Japanese bombers flying in 3 formations were intercepted over Lanzhou by 40 USSR volunteer and Chinese fighters taking off in small groups at 5-minute intervals. In the ensuing battle, nine Japanese bombers were shot down, killing 63 crew members, and one USSR pilot was wounded. Three days later on 23 February, the USSR fighters intercepted 57 Japanese bombers on bombing raids to the city and airfield of Lanzhou; the USSR fighters shot down six Japanese bombers and forced the Japanese to abandon the airfield target. In the battles of February, the Chinese pilots of the 17th Squadron fought alongside the USSR volunteers. Battle of Chongqing-Chengdu The massive IJAAF and IJNAF joint-strike terror-bombing campaigns and all-airwar which began in earnest on 3 May 1939 under the codename Operation 100 (100 号作战) as the Chinese Air Force was reorganizing from the Battle of Wuhan. In March 1939, the Chinese 4th Air Group, comprising 21st, 22nd, 23rd and 24th squadrons, was transferred to the airbase at Guangyangba for the defense of Chongqing. On 3 May, the 4th Air Group led by Group Commander Dong Mingde intercepted 54 Japanese bombers on their bombing raid to Chongqing, and shot down 7 Japanese bombers; deputy squadron commander Zhang Mingsheng (plane R-7153) was shot down and later died of his wounds. On 11 July, Chongqing was bombed by 27 Japanese bombers, which were met by eight I-15s led by Squadron Commander Zheng Shaoyu; Zheng's I-15 (No. 2310) had 38 bullets holes and I-15 (No. 2307) flown by pilot Liang Tianchen was shot down in flames. Battle of South Guangxi The battle over Nanning City, China last wartime access to seaport, including the Battle of Kunlun Pass. The final combat missions for Chinese-American volunteer pilot and fighter-ace Arthur Chin. In December 1939, the USSR fighter group, up to 50 planes under the command of S. P. Suprun was transferred to south Yunnan where Japanese air attacks on communications lines along the Chinese portion of the Burma Road had become more intense. Suprun's group participated in the Battle of South Guangxi, flying missions together with Chinese I-15 fighters from the 4th Air Group,