OP's Q was to flush out a (complex) aerospace product on time/price/Spec. We have responded with some diatribes about Waste-of-Space (we could have added Wastelands). Missing the point.
For-profit businesses do not relish R&D - high resource, low return. They want pea-in-pod volume at high margin - any colour you like as long as it's black. They don't need necessarily to make it, just to Prime it and take the mark-up - what does Daimler actually make in their Mercedes products? Early 787s had next-to-zero Boeing fabrication. So, Aerospace Prime Contractors now see themselves as integrators, not whittlers, of boxes and tin. If...a firm has the resilience to define its product, find the money, do it, and stand or fall on its market attractiveness, then, so. General Aviation offers many As, to OP's Q. But this is the Military Aviation thread.
Military Buyers think they know what they want and how to cause that to happen. In the 30 (>40!) years put forward by OP, only once has "success" accrued to a UK military Aero product: HSAL Hawk. The Spec. was clear, was put out to industry on a proposition of I pay, you do, was all-up contracted at a fixed package price; Customer left industry to get on with it. So that's the A to what is the secret.
But that cannot be read across to front line combat kit. If... Hawk had floundered in R&D, options (now COTS) were readily available. DoD and all the other hopeful Users for F-35s have no choice but to persevere and pay. While time goes by, they have no choice but to fund Customer-Initiated Change Proposals, scuttling the time/price basis of the original bid, because iPhone Old is of no use to our boys facing bad guys with iPhone Next. So, drift, bloat, until a Buyer says something NOW! please, not better, tomorrow, maybe.
RN made much of its management success in commissioning its Polaris/SSBNs on time (ordered 5/63, first patrol 15/6/68), on-Spec., on-budget. Well...but...this was as close to COTS as a weapon system could be (thank you, USN/DoD); it went to sea with a partial weapon load, and, as for on-budget, well what did that actually mean? “I hadn’t the
faintest idea (of hull build cost) so I took projected cost of Valiant (SSN) and doubled it.” (UK Polaris Executive Programme Manager...but I've lost the source!)