Theoretically you could make a PNP transistor by reversing the doping of an NPN transistor. The main problem is that boron diffuses rapidly, making it hard to fabricate a buried P-layer. Boron also has less solubility than phosphorus, making it hard to dope the emitter. Also, holes have only 1/3 the mobility of electrons, so PNP and NPN aren't symmetrical. To deal with these issues, PNP transistors are usually built with lateral construction (i.e. horizontally). The ring structure ensures that almost all of the carriers injected by the emitter are intercepted by the collector.
(This is based on The Art of Analog Layout, p280. I don't know all this doping stuff myself.)
It's worth noting that all of this mess was solvable, and made economical. Good (or at least fast) PNP and NPN transistors are now available together with modern complementary bipolar processes. These processes are in mass production and, these days, barely more expensive than the traditional ones even if they often involve fun things like SiGe.
(This is based on The Art of Analog Layout, p280. I don't know all this doping stuff myself.)