On May 18, 1942 at Rabaul at 5:55am eighteen G3M2 Nell bombers from the Imperial Japanese Navy (IJN) Genzan Kokutai led by Lt. Rokuo Nikaido took off from took off from Vunakanau Airfield armed with 60kg bombs on a mission to bomb 12 Mile Drome. Next, sixteen 4th Kokutai G4M1 Betty bombers led by Lt. Commander Hatsuhiko Watanabe took off from Vunakanau Airfield at 6:05am armed with 60kg bombs on a mission to bomb 7 Mile Drome. Escorting, nine A6M2 Zeros from the Tainan Kokutai took off from Lakunai Airfield at 6:25am but two Zeros aborted due to mechanical problems, leaving only seven on the mission.
Due to their different cruising speeds, the two different bomber formations proceeded separately to Port Moresby. Later, eleven A6M2 Zeros from the Tainan Kokutai took off from Lae Airfield with FPO2c Masayoshi Yonekawa aborted for mechanical reasons with the other ten rendezvousing with the Nells over Ioma at 9:15am and were spotted by Australian spotters who provided an early warning to Port Moresby via radio.
Meanwhile at Port Moresby, P-39 and P-400 Airacobras from the 36th Fighter Squadron and 35th Fighter Squadron plus attached aircraft from the 39th Fighter Squadron and 40th Fighter Squadron took off to intercept an incoming formation of G4M1 Betty bombers. The precise number of Airacobras involved in the scramble is unclear, but was either thirty-two (24 P-39s and 8 P-400) or only twenty-nine. One of the intercepting pilots was Lt. Harvey J. Scandrett.
At 8:40am, the Betty formation sighted eighteen enemy aircraft approaching and successfully bombed at 8:43am, hit the northwest end of the runway and dispersal area damaging water supplies, telephone lines, tents, a mess hall and two trucks from the Australian Army 17th anti-tank regiment, C Troop. At 9:00am the Nell formation bombed 12 Mile Drome hitting the runway, damaging huts and destroying two parked Airacobras and damaging another Airacobra.
After their bomb run, the Betty formation was intercepted by the Airacobras and shot down G4M1 Betty piloted by Oyama (KIA). Missing was P-39F 41-7191 (MIA). Damaged was G4M1 Betty piloted by Inoue and six other Bettys suffered minor damage but returned safely. Afterwards, escorting Zeros from the 2nd Shotai A6M2 Zeros engaged the Airacobras and prevented them from making further attacks.
The Japanese formation returned to base. Returning, G4M1 Betty piloted by Inoue crash landed at at Lae Airfield, the others returned safely to Rabaul. All the Nells and Zeros returned safely. The Zeros claimed three Airacobras shot down. The Betty bombers also claimed four Airacobras shot down. On the American side, P-39F 41-7191 went missing and several other Airacobras returned with damage.
Kodochosho, Genzan Kōkūtai, May 18, 1942
Kodochosho, 4th Kōkūtai, May 18, 1942
Kodochosho, Tainan Kōkūtai, May 18, 1942
36th Squadron Narrative History
Mitsubishi Type 1 Rikko 'Betty' Unit of World War 2 page 41
Eagles of the Southern Sky pages 139-140
War Diary 1942 does not mention any air raid on May 18, 1942
Thanks to Edward Rogers for additional information