Bubble.

This is coolbert:

From a comment to the blog by Maximex:

"Similarly, the P-51B Mustang became a good fighter only after assembly of the Rolls-Royce Merlin engine and Spitfire bubble canopy  . . . that plane was the P-51D model, 'the normal' mustang" [this comment of Maximex slightly edited!]

And Maximex is correct. Those early versions of the P-51 NOT having the bubble canopy. That modification [bubble canopy] as seen in the "D" variants of the Mustang onward allowing for much greater and improved visibility for the allied combat aviator.

 

An early version of the P-51 sans bubble canopy.

"The tenth production P-51B 43-12102, prototype for the P-51D, showing the modified rear fuselage and new canopy and windscreen."

"Following combat experience the P-51D series introduced a 'teardrop', or 'bubble', canopy to rectify problems with poor visibility to the rear of the aircraft"

 

That classical version of the P-51 Bubble canopy and drop tanks.

See the enemy and surprise him before you are seen and surprised!!

As from the dicta of Boelcke.

"Surprise: getting the first shot before one's opponent is prepared to return fire was the 'safest' and preferred method for attack. Most air victories were achieved in the first pass. Without all-seeing devices like radar, a pilot could approach his foe stealthily, using clouds, haze or even using the enemy aircraft's own wings or tail to conceal his approach. The glare of the sun, especially, provided an effective hiding spot."

As it was in the Great War so it was it too in the Second World War and so will it be ALWAYS!

coolbert.