Welch resigned from the United States Army Air Forces as a major in 1944, and became a test pilot for North American Aviation.

Taking off with only .30-cal ammunition in the wing guns, Welch claimed two kills of Aichi D3A Val dive bombers over Ewa Mooring Mast Field.

[3] The first Japanese aircraft was only damaged and made it back to its carrier, while the second was finished off by Ken Taylor, shortly before he landed at Wheeler Field to get .50-cal ammo for his two cowl guns.

On his second sortie, Welch shot down a Val (which was behind Ken Taylor, and crashed in the community of Wahiawa) then one Mitsubishi Zero fighter about 5 miles west of Barbers Point.

Welch shot down a Zero and two Aichi D3A “Val” dive bombers on December 7, 1942, the first anniversary of Pearl Harbor.

Between June 21 and September 2, 1943, flying a P-38H, Welch shot down nine more Japanese aircraft: two Zeros, three Ki-61 Tonys, three Ki-43 Oscars, and one Ki-46 Dinah.

On November 1, North American, with the aid of captured German technology, proposed and was given permission for a major redesign of the XP-86 to a 35-degree, swept-wing configuration.

[10] When he reached a low-fuel state, he elected to land on Muroc Lake Bed without a fully extended nose gear.

Upon touchdown, in a nose-high attitude, Welch cut the engine, and as the XP-86 slowed, the nose gear snapped down and locked.

[8] Secretary of the Air Force[N 2] Stuart Symington had instructed North American that they were not, under any circumstances, to break the sound barrier before the X-1 achieved this milestone.

In his book Aces Wild: The Race for Mach 1 (1998), fellow North American test pilot Al Blackburn speculates that Welch may have broken the sound barrier two weeks before Chuck Yeager in an early flight of the XP-86 prototype.

[12] Robert Kempel, author of The Race For Mach 1 contradicts the claim, contending for Welch's aircraft to break the sound barrier with an underpowered engine was impossible.

[14] Blackburn, however, maintains that a record on the Muroc radar theodolite, of the two flights Welch made on November 13, 1947, indicated supersonic flights, as well, noting 20 minutes before the X-1 broke the record, a sonic boom was heard over the desert, centered on the Happy Bottom Riding Club,[N 3] a dude ranch restaurant and hotel operated by Pancho Barnes.

[13] Welch went on to work as chief test pilot, engineer, and instructor with North American Aviation during the Korean War, where he reportedly downed several enemy MiG-15 Fagots while "supervising" his students.

After the war, Welch returned to flight testing; this time in the F-100 Super Sabre, with Yeager flying the chase plane.

Stability problems were encountered in the flight test program, and on Columbus Day, October 12, 1954, Welch's F-100A-1-NA Super Sabre, AF Ser.

52-5764, disintegrated during a 7-G pullout at Mach 1.55 from 45,000 ft (13,500 m) and crashed in Rosamond Lake in the Mojave Desert about 45 miles (72 km) north of Los Angeles.

His decorations include:[19] The President of the United States of America, authorized by Act of Congress, July 9, 1918, takes pleasure in presenting the Distinguished Service Cross to Second Lieutenant (Air Corps) George Schwartz Welch, United States Army Air Forces, for extraordinary heroism in connection with military operations against an armed enemy while serving as Pilot of a P-40 Fighter Airplane in the 47th Pursuit Squadron, 18th Pursuit Group, Hawaiian Air Force, in action over the Island of Oahu, Territory of Hawaii and waters adjacent thereto, on 7 December 1941.

Lieutenant Welch's initiative, presence of mind, coolness under fire against overwhelming odds in his first battle, expert maneuvering of his plane, and determined action contributed to a large extent toward driving off this sudden unexpected enemy air attack.