The Spitfire IXe was a development of the standard Spitfire IX that inverted the position of the 20mm cannons and replaced the outboard .30 caliber machine guns with .50 caliber weapons in the wing position formerly occupied by the cannons.
While the IXe wing entered production in the Spring of 1944, many Spitfires thought to be examples of this sub-type are actually earlier Spitfires, with a “field modification” to change the gun position, without the inner .50 caliber weapon, which was done due to the fact that with Spitfires being equipped with bomb racks for the fighter-bomber role in the invasion of France, having a 250-lb bomb and a 20mm cannon in the same wing location put too much strain on the wing structure.
Such an airplane was MK392, the Spitfire flown by Wing Commander Johnny Johnson when he returned to combat operations in March 1944 to assume command of a second Canadian Spitfire wing, 144 Wing in 83 Group of the 2nd Tactical Air Force which became 144 RCAF (Fighter) Wing, a fighter/bomber unit comprised of 441, 442 and 443 Squadrons. MK392, the Spitfire Johnson flew with 144 Wing has now been identified as being originally produced as a Spitfire IXc. In June 1944, Johnson had an MU modify the airplane by changing the gun positions, which was not difficult since MK392 used the large teardrop-shaped “universal gun cover.” All the Spitfires in 144 Wing, which did include some early IXe airplanes that were not equipped with the .50 caliber machine guns (which only made their appearance in the fall of 1944 due to production delays of the weapons), removed the outer .30 caliber weapons to save weight since the rifle-caliber weapons were not considered particularly effective either in strafing or air combat.
Air Vice Marshal James Edgar Johnson, CB, CBE, DSO & Two Bars, DFC & Bar (9 March 1915 – 30 January 2001), nicknamed "Johnnie", was a Royal Air Force (RAF) pilot and flying ace.
Johnson was credited with 34 individual victories over enemy aircraft, as well as seven shared victories, three shared probable, 10 damaged, three shared damaged and one destroyed on the ground.
Johnson flew 700 operational sorties and engaged enemy aircraft on 57 occasions.
Included in his list of individual victories were 14 Messerschmitt Bf 109s and 20 Focke-Wulf Fw 190s destroyed making him the most successful RAF ace against the Fw 190. This score made him the highest scoring Western Allied fighter ace against the German Luftwaffe.
Johnson scored the bulk of his victories flying two Spitfires Mk. IX. The first one was EN398 JE-J, in which he shot down 12 aircraft and shared five, plus six damaged while commanding the Kenley Wing.
His second plane , MK392, was an LF Mk.IX, in which his tally increased by another 12 aircraft, plus one shared destroyed on the ground. He was to end the war flying another Spitfire Mk XIVe MV268.
As a Wing Leader Johnson was entitled to personal code letters and his aircraft were always marked JE-J.
Following the Normandy Invasion, 144 Wing became the first Allied unit to be based in Europe, and during the intense aerial battles over Normandy that summer, the wing became the top-scoring RCAF fighter wing of the war. W/C Johnson scored his 22nd-34th victories while leading the wing, with his final victory scored on September 27, 1944 over Nijmegen in support of Operation Market Garden.