A bubble canopy is a canopy made without bracing, which attempts to provide 360° vision to the pilot.
Bubble canopies have been in use since before World War II, with some experimental bubble canopy designs in the World War I era. The British had already developed the "Malcolm hood", which was a bulged canopy, but the British Miles M.20 was one of the first aircraft designs to feature a true one-piece sliding bubble canopy. Although that aircraft never went into production, the concept of the bubble canopy was later utilised on other British aircraft, such as the Hawker Typhoon and Tempest. It was also later fitted to the P-51 Mustang and P-47 Thunderbolt amongst others. A well-framed version of an all-around vision canopy was also used on the Mitsubishi A6M Zero Japanese naval fighter, and different designs with much less framing than the "Zero" had, were used on the Imperial Japanese Navy Air Service's Nakajima Ki-43 Oscar and Nakajima Ki-84 Frank land-based fighter planes.
The Bell 47 helicopter was the first production helicopter certified for civilian use in the United States, and in its Model 47D version, pioneered the "soap bubble"-style canopy for light helicopters — as named by its designer, Arthur M. Young — that it and the 47G model were to become famous for.
The purpose of a bubble canopy is to give a pilot a much wider field-of-view than flush, well-framed "greenhouse" canopies used on early World War II aircraft, such as those seen on early models of the F4U, P-51, the Soviet Yak-1 and earlier, "razorback" P-47 fighters, all with dorsal "turtledecks" integral to their fuselage lines, which left a conspicuous blind spot behind the pilot that enemy pilots could take advantage of to sneak up on an aircraft.
The open-cockpit design combat aircraft of World War I had narrow fuselages, which often were not tall enough to block visibility to the rear, especially with seating positions that generally elevated the pilot's head well above the cockpit's edges. As planes became larger, heavier and faster, designs had to be made stronger, which often meant a taller rear fuselage, but designers tried to maintain the narrow fuselage for visibility.
However, as speed continued to increase, it became necessary to enclose cockpits – and this, in turn, streamlined aircraft so that they were faster. Increased "G-loading" during maneuvers forced pilots to wear tight, restrictive shoulder harnesses, and armor plating began to be installed to protect pilots from projectiles coming from behind.
Unfortunately, these changes denied a pilot the ability to twist around and look directly behind (known as "Checking Six," or looking at the "Six O'clock" position directly to the rear). Mirrors offered some help, but had a narrow field of view.
Prior to bubble canopies, some aircraft, such as the P-40 Warhawk, featured a hybrid flush canopy design, combining a narrow rear fuselage with a glass enclosure conforming to the shape of a full-width fuselage - these often had a pair of recessed panels (one per side, behind the openable canopy) in the dorsal "turtledeck" structure, faired-in with framed glazing that was flush to the fuselage surface. This provided increased visibility while still allowing a pilot to keep the canopy closed for greater performance. Examples of such "recessed" rear vision designs were the "greenhouse"-canopied original F4U-1 Corsair as well as the P-40.
The bulged Malcolm hood, used for the Spitfire, F4U Corsair, and P-51B and -C Mustangs was another hybrid. While not offering as much visibility to the rear as the P-40 enjoyed, it allowed a pilot more visibility than a flush canopy would.