Boeing submitted the prototype for the B-29 long-range heavy bomber to the Army in 1939, before the United States entered World War II.The B-29 recorded its first flight on 21 Sep 1942. It flew its first combat mission on 5 June 1944.
At the time, the B-29 was a technological marvel.
A very advanced bomber for this time period, it included features such as a pressurized cabin, an electronic fire-control system, and remote-controlled machine-gun turrets.When was it designed?
[Wikipedia, B-29 Superfortress]
One of the most advanced bombers of World War II, design of the Boeing B-29 began in the late 1930s.Taking "late 1930s" to be 1936, from inception to combat, this plane was 8 years in the making.
[World War II: Boeing B-29 Superfortress]
How long does it take modern combat aircraft to become operational?
"The JVX [Joint-service Vertical take-off/landing Experimental] aircraft was designated "V-22 Osprey" on 15 January 1985.FIFTEEN years from inception to operation - and "operation" here is merely training.
Marine Corps crew training on the Osprey has been conducted by VMMT-204 since March 2000."
[Wikipedia, V-22 Osprey, emphasis added]
How has our military, the Pentagon, become so reliant on high-tech gadgets that take forever to develop, cost tons of $$$, and may or may not perform in the field?
We entered WWII with little and emerged with much - in no more than 10 years.
Today - in peacetime - we devote twice the time to develop weapons systems we don't need.