As mentioned in the previous chapter, Hiryū was the second aircraft
carrier included in "The Second Naval Armaments Supplement Program" of
1934. Originally both carriers were supposed to be sister vessels, but
the number of design modifications introduced during the construction of
Sōryū resulted in many differences between the two. According to the
original plans Hiryū was to be completed a year after Sōryū, but her
construction (similarly to her predecessor) suffered delays caused by
two key factors. The first one was the implementation of the lessons
learned during the reconstruction of Kaga, which was going on
simultaneously with Hiryū's construction. Then there was new data
available from the early service days of Sōryū, which exposed some of
the design's drawbacks and weaknesses. The number of issues popping up
"along the way" was further increased by the Fourth Fleet Incident and
by Japan's withdrawal from the previously signed naval treaties.
Considering all those issues, it is not hard to imagine the inevitable
impact they had on Hiryū's original design and construction schedule.
The greatest source of delays was undoubtedly the aftermath of the
Fourth Fleet Incident, which forced the Navy Aviation Bureau to
introduce changes in the design of the second carrier. After the new
requirements had been implemented, Hiryū's final design (known as the
"Basic Project G-10") finally emerged.
