That is a very nice set of aircraft that you have to build, David.
Your start is looking great already, very detailed.
Especially for 1/72

1/72 Airfix Spitfire MK 1a – After completing the cockpit I assembled the fuselage. The exhaust from the kit was modified by drilling out the pipes. Since these all are going to be built wheels up, I installed the landing gear up. The underside was then base coated with sky gray. The topside was base coated with RAF dark green and dark earth for the requested scheme. The client included a specific set of decals for the scheme as well. With decals applied and some mild weathering the aircraft was top coated. I polished the canopy and painted the frame then installed it. Right behind the canopy is a red navigation light. I removed the molded in one and replaced it with a clear lens. Finally I used EZ-line for the antenna. This now completes the MK-1a Spitfire! Now starting on the MK-1a Spitfire that will be modified to the MK-II Spitfire using the 3D Kits conversion parts. For now here are the final photos of the MK-1a.

You can see all the photos and details in my build log at https://davidsscalemodels.com/build-log/1-72-multi-aircraft-commissioned-build/

9 attached images. Click to enlarge.

Great job, David @davids_models!
I agree with John @johnb, about the exhaust drilled looks.

Very detailed paintwork, David.
Looking great.

1/72 Airfix Spitfire MK II – This was stopped as the client wanted a slight change to the scheme. He wanted to use Mission Models paint colors for this. So this build is paused until the paint arrives. While waiting on the paint, I started on the Tamiya P-51D.
1/72 Tamiya P-51D Mustang – Since this is mostly straight out of the box this build went very smoothly. The cockpit was detail painted and lightly weathered. I added some of the cabling to the equipment section behind the pilot seat. This kit did not come with a pilot figure so I used one of the PJ Production pilots I had. The figure fit perfectly in the seat. The cockpit was mounted in the fuselage and the rest of the fuselage assembled. The tail wheel doors were molded with the fuselage in the open position. Since this is being built “wheels up” I cut off the open doors and trimmed the doors to fit closed. The fuselage was sprayed with gloss aluminum with the nose and spinner painted blue. Next the decals were added and a gloss top coat was sprayed on. For the recognition lights (see https://davidsscalemodels.com/tips-and-tricks/recognition-lights/ ) I drilled a divot where the lights go, Painted the inside chrome silver, then filled the divot wit acrylic gel. Once dry they were painted using Tamiya clear paint for each color. The canopy was then polished, painted, and installed. This completed the P-51. This makes the Multi-Aircraft build 50% completed.

The paint for the MK-II has arrived so I will finish up the MK –II then start working on the MK IXc Spitfire.

You can see all the photos and details in my build log at https://davidsscalemodels.com/build-log/1-72-multi-aircraft-commissioned-build/

10 attached images. Click to enlarge.

That's a great looking P-51, David!
I loved the clear gel wingtip light interpretation!

1/72 Eduard Spitfire MK IXc Late Version –This is the fourth out of five aircraft. This Eduard kit is the ProfiPACK which includes photo etch parts for the cockpit. The cockpit was assembled and I used another PJ Production RAF pilot since the kit did not supply one. The photo etch instrument panel had three sections as well as the compass. The cockpit had separate walls so I was able to add some details and paint. The resin figure had the seat belts molded on. These were partially trimmed off and the photo etch seat belts were modified to match the molded seat belts and I had to make the buckle for the belt to hook together. One set of seat belts mounted to the back of the seat and there is a second set that mounted to the back wall of the cockpit.
With the fuselage together, I added some more photo etch details to the underside. The bottom was then base coated in Sky Grey. The camouflage was then done with RAF ocean grey and RAF dark green. The spinner and fuselage stripe were painted with duck egg. The kit decals were applied and I added some EZ-Line for the antenna. The canopy was polished, painted, and installed which completed this aircraft. The Eduard kit has a little better fit and much better quality decals than the Airfix kits. The photo etch accessories were a nice feature. Even with a pilot and canopy closed the instrument panel details can be seen. With this one finished I am now finishing up the MK II Spitfire then I just need to build the fifth (and last) one of this multi-aircraft build which is the 1/72 Academy F-35A.

You can see all the photos and details in my build log at https://davidsscalemodels.com/build-log/1-72-multi-aircraft-commissioned-build/

11 attached images. Click to enlarge.