The P-51H was designed to British stress standards (ie lower factor of safety) like the Spitfire. So, if the Spitfire was fragile NAA were'nt all that concerned. Yes they were! It was only after they had new manufacturing methods of riveting the skin and a very much stronger alloy than that used in any Spitfire. The H was built using American standards for strength, using British thickness of skinning.
Also, when did a P-51 with the same power outclimb a Spitfire?
When the two planes are at the same weights and effective powers. Similar engines and take off weights. Look up the rate of climb on this chart;
http://www.wwiiaircraftperformance.org/mustang/p-51h-altperf-91444.jpg ! I've never heard of any Spitfire with a RoC over 6,300 FPM? Even the Mk-XIV was 3,600-5,400 FPM depending on mission and sources quoted!
See Wiki;
Specifications (Spitfire Mk XIV)[edit]
A Spitfire XIVE RN119 of 402(RCAF) Squadron in March 1945.
General characteristics
Crew: one pilot
Length: 30 ft (9.14 m)
Wingspan: 36 ft 10 in (11.23 m)
Height: 10 ft (3.05 m)
Wing area: 242.1 ft2 (22.48 m2)
Airfoil: NACA 2209.4(tip)
Empty weight: 6,578 lb (2,984 kg)
Loaded weight: 7,923 lb (3,593 kg)
Max. takeoff weight: 8,488 lb (3,850 kg)
Powerplant: 1 × Rolls-Royce Griffon 65, supercharged V12 engine , 5-bladed Jablo-Rotol propeller, 2,050 hp (1,528 kW) at 8,000 ft (2,438 m)
Performance
Maximum speed: 448 mph, (391 kn, 717 km/h)
Combat radius: 400 nmi (459 mi, 740 km)
Ferry range: 950 nmi(1,090 mi, 1,815 km)
Service ceiling: 43,500 ft (13,258 m)
Rate of climb: 3,650 ft/min (18.5 m/s)
Wing loading: 32.72 lb/ft2 (159.8 kg/m2)
Power/mass: 0.24 hp/lb (0.42 kW/kg)
Armament
Guns: ** 2 × 20 mm (0.787-in) Hispano Mk II cannon, 120 rpg. 4 × 0.303 in (7.7 mm) Browning machine guns, 350 rpg. Replaced by 2 x .50 in (12.7 mm) M2 Browning machine guns 250 rpg Mk XIVE.
Bombs: 2 × 250 lb (113 kg) bombs