A new book would be more than welcome! Warren Bodie's works and the old Sentry AirPower/Wings articles portray Kelsey as one of the greatest persona in the Pre-War Air Corps.
I don't question Kelsey role in the creation of Interceptor RFP to create opportunity for RFP and development of the P-38 and P-39. That said, under hs leadership nothing else but the P-40 and P-61 emerged from Wright Field Materiel Command solicitations when he was in charge of Fighter development.
With respect to NAA, he (Kelsey) made several claims which were captured by Bodie and extended by Ethell.
1.) He claimed to have been instrumenal in NAA interest in NACA Laminar Flow airfoil, 2.) He claimed to be involved during the actual selection of the future NAA/NACA 45-100 airfoil and equally being the guiding hand behind NACA dvelopment of 66 series airfoil, which later was tested in XP-51F series and P-51H production wing.. Per the latter claim, he (Kelsey) stated as quoted bt Ethell "Still behind the scenes, but well aware of what was happening, Echols sent Kelsey to Langley to make sure that North American received as much data as possible from Curtiss and from the NACA tests". "The records show all this happened", recalls Kelsey, "without anybody at Wright Field having the foggiest notion of what was going on. We had to stay out of it, because it was a British procurement".
Unfortunately for Kelsey's credibility his timeline recollections are so screwed up as to be ludicrous. First, Horkey suggested strongly (To AFPC Technical team) that a derivative of the above referenced NACA 45-125 Laminar Flow wing conception as tested was promising but impossible to reproduce with existing manufacturing methods while presenting the original P-509 in NY in mid March 1940. The decision for two branches for airfoil was offered - one with NACA 2412 (Spit wing) and one 'proprietary NAA Low Drag High Speed airfoil'. The go-ahead to start on what would become NAA/NACA 45-100 airfoil was granted by Kindelberger to Schmued/Horkeywhen the LOI was signed by Lord Self on 10 April. The work started and well on the way to conclusion before the NAA submitted the request to expedite '50B' (NA-73) to Materiel Command on 20 April. Echols noted that the proposed Specifiation for 50B contained an Allison engine which for any country other than Great Britain would have been rejected. THERE WERE ZERO DETAILS REGARDING THE AIRFOIL OTHER THAN THE NACA 2412 and the NAA High Speed Low Drag airfoil - with no other specificity.
In other words Kelsey was never in the loop regarding Horkey'y team activities sorting out the airfoil with Kutta-Joukowski/Theodorsen transformations - and there is no record of Materiel Command ever in the communication cycles between NAA and NACA technology personnel in Los Angeles.
In my deep dive research, the only correspondence between Mat'l Command and NAA were the discussions about 'donating two XP-51 articles from the first 10, agreement that #4 and #10 were the specific two, and that AAC/AAF investigated the delivered P-40 and XP-46 data to sort out whether taxpayer funding was used for wind tunnel data that Atwood purchased from Burdette Curtiss at the request of Sir Henry Self.
Kelsey could not have even claimed that he was 'helping' NAA in research data from NACA on the XP-40, P-40 or XP-46 attempts at 'meridith effect' radiator cooling as none of the above were tested and evaluated for pressure distributions surrounding the radiator.
2.) He claimed that Kelsey was instrumental in 'manuevering the NAA Mustang procurement for A-36' due to his vision for the Mustang, as there was never a need for a dive bomber'.
Kelsey never read the memo that Arnold and Spaatz were satisfied that the Mustang was better by far than P-40 and P-39 and on par with P-38/P-47 and that Arnold was under pressure to develop Ju 87 Stuka capability by Sec'y of War in 1940, and that the emerging discontent with Materiel Cmd development and procurement for suitable Fast Attck/CAS/battlefield air superiority 'pursuit' was a strong force at AAF HQ. Echols was crushed in his battles to preserve his turf with MG Fairchild and CAS Directorate at HQ.
More ludicrous is that Kelsey had been in England two months in his 8th AF assignment to move P-38s to ETO before the A-36 selection/contract process began!
Kelsey was offshore as all the turf batteles for A-36, then follw on P-51A, then conversion to P-51B were fought between HQ - Planning and Requirements and Echols.
Even when he was tasked to command Air Technical Support for 8th AF, along with Cass Hough, he seemed to devote much of his energies to sort out the P-38, including procuring a P-38 to send to R-R for conversion to the 'silver chalice' P-38K and got crushed by Doolittle/Spaatz.
Summary - Ben Kelsey never supported NAA or the P-51 - ever - over the P-38. His 'recolections regarding contributions are not documented nor do they conform to timelines. Ben Kelsey and Oliver Echols tried to explain away the Wright Field indifference to flight testing. In fact the NACA were gleefully granted XP-51 by Mat'l Command and Tested Expertly for several months while XP-51 #2 sat untested through April 1942 (according to Chilton 4 hours flight time from delivery in December 1941)