During Brandal Gareth's attempt to secede from the Lyran Alliance in 3057, he introduced to the Inner Sphere what he thought would be the next generation of war machinery, the Defiance.
It was an utter flop.
A poorly conceived and designed heavy mech, the Defiance had massive firepower but nowhere near the heat sink capacity to use them. Derided as a white elephant, the LAAF stopped ordering the mech almost immediately after the results of its first, disastrous outing against the Jade Falcons.
Casting for something to do in 3069, Defiance Industries decided that to do nothing about the design was a bit of a waste, and reworked it to make it a viable design.

After they downgraded the ER PPCs to Defiance 6 model large lasers, and the Defiance Disintegrator to a Defiance Destroyer autocannon, and removed the MASC system, enough weight was saved that they could replace the XL engine with a light engine.
With some spare weight left, they upgraded the SRMs to streak models, and the medium pulse lasers with Ramtech extended range models.
Some more freezers were added to handle the heat output, and the mech is capable of prolonged combat without overheating.

The new Defiance has been tentatively placed on the Jade Falcon border for evaluation. Thankfully, most of the initial reports are promising. Defiance Industries thinks it can start full scale production as soon as the LCAF gives the green light.