Stretch forming was not unknown in the industry at the beginning of WW2. The 150 ton stretch press at Castle Browmwich is surely for sheet stock. The 20 or so cylindrical sized caps I believe are the hydraulic cylinder heads, probably for the clamping jaws to hold the sheet but possibly to raise the forming die.
The Me 109:
EzineArticles Submission - Submit Your Best Quality Original Articles For Massive Exposure, Ezine Publishers Get 25 Free Article Reprints
"
The rear section of the fuselage (the section immediately behind the pilot' seat and extending to the front of the tail) was also unique. To explain, first visualise a stack of cups. The rear section or in German, the "Rumpf" is made up of parts that nestle together like a stack of cups. Each section including what would normally be the former or main structural part forming the shape is made of a single sheet of aluminium. Each section was formed on a stretch press with the former being created out of some clever folds to provide strength. To be precise every second section was formed on a stretch press. So, to visualise, keep every second cup and replace every alternate one with a sheet wrapped around the bottom of each cup - that is what the back end of the Bf 109 is like. It was innovative, strong and it greatly simplified production by removing a lot of fiddly framework."
Martin B-26 Marauder:
LiTOT: The Martin Marauder B-26
"
The general production setup called for extensive use of spotwelding in secondary structure to eliminate, where possible, the more costly and time-consuming process of riveting. The stretch press, used by the automobile industry at the time of World War I to form fenders, was reintroduced to shape convex-curved pieces of aluminum skin, four times the average size in other aircraft. These large sheets of skin were employed in the fuselage and cowling to eliminate the multiplicity of stringers, rivets and other small detail parts normally associated with a fuselage of this size and length. Hundreds of castings, forgings and plastic parts were used to simplify mass production methods and eliminate the use of critical materials. Thus, the B-26 uses more castings, forgings and other miscellaneous parts adapted to intensive production methods, fewer rivets and detail parts, than any comparable airplane. Some of the new design elements in the Marauder follow:
WING ARRANGEMENT — It was decided to depart from the usual procedure of keeping wings integral from nacelle to nacelle by breaking the wing panels at the sides of the fuselage. This entailed the eventual design of the "dumbbell" lower spar chord forging to eliminate the use of heavy and costly fittings. Manufacture of this forging, the largest aluminum type ever made, required five sets of dies, each weighing more than two tons. Thus the wing and its engine and nacelle is an entire assembly. The center section was eliminated. Each wing is attached to the dural forgings at the wing roots, which serve as anchors for the attaching bolts. This arrangement facilitates assembly and replacement.
FUSELAGE — Its construction around a very strong keel made for almost perfect streamlining, and extra strength in crash landings. B-26 crews nearly, always walk away from a forced landing. This same keel was also excellent as an anchor for the external torpedo rack. There are no stringers, just skin and forming frames — five compartments in three major sections."
The Pe-2 also had this kind of structure but was regarded as a little weak.
The below is from "Martin B-26 Marauder" by Martyn Chorlton (Osprey Publishing)
The Marauder was designed and built from the outset to be a mass-
produced aircraft. Martin engineers confidently pre-empted a large order
being received from the USAAF as the world situation continued to
deteriorate. Martin also accurately predicted that the workforce required
to build the bomber would have no, or very little, experience of building
aircraft.
To help achieve the task of building a complex flying machine, the
B-26 was put together from approximately 650 minor sub-assemblies,
which were then joined to create 32 major sub-assemblies, and the final
aircraft. All of the Marauder's frames and stringers were pre-drilled in
their jigs and, once these components were delivered to the shop floor,
the task of riveting was made all the easier.
The Marauder's fuselage was covered in 63 pieces of metal skin, each
with its own compound curvature. Each piece of skin was produced on a
stretch press with a high degree of accuracy and consistency that made
the job of producing the B-26's streamlined fuselage quicker than be-
spoke panel beating.
Wing parts were also produced with the same degree of accuracy and
consistency thanks to Martin working closely with a milling machine
manufacturer to develop a machine with a travelling head and a 30ft bed.
Every part of the wing was produced to be foolproof, basically meaning
that the parts could only be fitted in one way — the correct one!